Pulsed Field Ionization-photoelectron Spectra Research Articles

A HIGH-RESOLUTION VACUUM ULTRAVIOLET LASER PHOTOIONIZATION AND PHOTOELECTRON STUDY OF THE CO ATOM

ABSTRACT We have measured the vacuum ultraviolet–photoionization efficiency (VUV–PIE) spectrum of Co in the energy range of 63,500–67,000 cm−1, which covers the photoionization transitions of Co(3d74s2 4F9/2) Co+(3d8 3F4), Co(3d74s2 4F7/2) Co+(3d8 3F3), Co(3d74s2 4F9/2) Co+(3d8 3F3), Co(3d74s2 4F9/2) Co+(3d8 3F2), and Co(3d74s2 4F9/2) Co+(3d74s1 5F5). We have also recorded the pulsed field ionization photoelectron spectrum of Co in the same energy range, allowing accurate determinations of ionization energies (IEs) for the photoionization transitions from the Co(3d74s2 4F9/2) ground neutral state to the Co+(3F J ) (J = 4 and 3) and Co+(5F5) ionic states, as well as from the Co(3d74s2 4F7/2) excited neural state to the Co+(3d8 3F3) ionic state. The high-resolution nature of the VUV laser used has allowed the observation of many well-resolved autoionizing resonances in the VUV–PIE spectrum, among which an autoionizing Rydberg series, 3d74s1(5F5)np (n = 19–38), converging to the Co+(3d74s1 5F5) ionic state from the Co(3d74s2 4F9/2) ground neutral state is identified. The fact that no discernible step-like structures are present at these ionization thresholds in the VUV–PIE spectrum indicates that direct photoionization of Co is minor compared to autoionization in this energy range. The IE values, the autoionizing Rydberg series, and the photoionization cross sections obtained in this experiment are valuable for understanding the VUV opacity and abundance measurement of the Co atom in stars and solar atmospheres, as well as for benchmarking the theoretical results calculated in the Opacity Project and the IRON Project, and thus are of relevance to astrophysics.

State-to-state Photoionization Dynamics of Vanadium Nitride by Two-color Laser Photoionization and Photoelectron Methods

We have conducted a two-color visible-ultraviolet (VIS-UV) resonance-enhanced laser photoionization and pulsed field ionization-photoelectron (PFI-PE) study of gaseous vanadium mononitride (VN) in the total energy range of 56900–59020 cm−1. The VN molecules were selectively excited to single rotational levels of the intermediate VN(D3Π0, v′=0) state by using a VIS dye laser prior to photoionization by employing a UV laser. This two-color scheme allows the measurements of rovibronically selected and resolved PFI-PE spectra for the VN+(X2Δ; v+=0, 1, and 2) ion vibrational bands. By simulating the rotationally resolved PFI-PE spectra,J+=3/2 is determined to be the lowest rotational level of the ground electronic state, indicating that the symmetry of the ground VN+ electronic state is 2Δ3/2. The analysis of the PFI-PE spectra for VN+ also yields accurate values for the adiabatic ionization energy for the formation of VN+(X2Δ3/2), IE(VN)=56909.5±0.8 cm−1 (7.05588±0.00010 eV), the vibrational frequency ωe+=1068.0±0.8 cm−1, the anharmonicity constant ωe+χe+=5.8±0.8 cm−1, the rotational constantsBe+=0.6563±0.0005 cm−1 and αe+=0.0069±0.0004 cm−1, and the equilibrium bond length, re+=1.529 Å, for VN+(X2Δ3/2); along with the rotational constantsBe+=0.6578±0.0028 cm−1 and αe+=0.0085±0.0028 cm−1, and the equilibrium bond length re+=1.527 Å for VN+(X2Δ5/2), and the spin-orbit coupling constant A=153.3±0.8 cm−1 for VN+(X2Δ5/2,3/2). The highly precise energetic and spectroscopic data obtained in the present study are valuable for benchmarking the predictions based on state-of-the-art ab initio quantum calculations.

Rovibronically selected and resolved two-color laser photoionization and photoelectron study of titanium monoxide cation

Two-color visible-ultraviolet (VIS-UV) resonance-enhanced laser photoionization and pulsed field ionization-photoelectron (PFI-PE) study of gaseous titanium monoxide (TiO) in the total energy range of 55,000-57,320 cm(-1) has been conducted. The TiO molecules were selectively excited to single J' rotational levels of the intermediate TiO*(B(3)Π1, ν' = 0) state by using a VIS dye laser and then ionized by using another UV laser. This two-color photoexcitation method has allowed the measurement of cleanly J(+)-resolved PFI-PE spectra for the TiO(+)(X (2)Δ(5/2,3/2); v(+) = 0, 1, and 2) vibrational bands. By simulating the rotationally resolved PFI-PE spectra, J(+) = 3∕2 is determined to be the lowest rotational level of the ground electronic state, confirming that the symmetry of the TiO(+) ground state is (2)Δ(3/2). Irregular intensity patterns for rotational PFI-PE peaks that deviated from the regular patterns that favor the rotational transitions with small change of the core rotational angular momentum, are observed. This observation is indicative of strong perturbations of the PFI-PE rotational transitions, possibly by a channel-coupling mechanism. The analysis of the PFI-PE spectra yields highly precise values for the adiabatic ionization energy of TiO [IE(TiO) = 55 005.4 ± 0.8 cm(-1) (6.81980 ± 0.00010 eV)], and the vibrational frequency (ωe(+) = 1056.1 ± 0.8 cm(-1)), the anharmonicity constant (ωe(+)χe(+) = 4.4 ± 0.8 cm(-1)), the rotational constants (B(e)(+) = 0.5613 ± 0.0009 cm(-1) and αe(+) = 0.0029 ± 0.0008 cm(-1)), and the equilibrium bond length (r(e)(+) = 1.583 Å) for the TiO(+)(X (2)Δ(3/2)) ground state, the vibrational frequency (ωe(+) = 1058.4 ± 0.8 cm(-1)), the anharmonicity constant (ωe(+)χe(+) = 5.1 ± 0.8 cm(-1)), the rotational constants (B(e)(+) = 0.5715 ± 0.0007 cm(-1) and αe(+) = 0.0030 ± 0.0004 cm(-1)) and the equilibrium bond length (r(e)(+) = 1.568 Å) for the excited spin orbit state TiO(+)(X (2)Δ(5/2)), along with the spin-orbit coupling constant (A = 105.9 ± 0.2 cm(-1)) for TiO(+)(X (2)Δ(5/2,3/2)).

Rovibronically selected and resolved two-color laser photoionization and photoelectron study of cobalt carbide cation

We have conducted a two-color visible-ultraviolet (VIS-UV) resonance-enhanced laser photoionization efficiency and pulsed field ionization-photoelectron (PFI-PE) study of gaseous cobalt carbide (CoC) near its ionization onset in the total energy range of 61,200-64,510 cm(-1). The cold gaseous CoC sample was prepared by a laser ablation supersonically cooled beam source. By exciting CoC molecules thus generated to single N' rotational levels of the intermediate CoC∗((2)Σ(+); v') state using a VIS dye laser prior to UV laser photoionization, we have obtained N(+) rotationally resolved PFI-PE spectra for the CoC(+)(X(1)Σ(+); v(+) = 0 and 1) ion vibrational bands free from interference by impurity species except Co atoms produced in the ablation source. The rotationally selected and resolved PFI-PE spectra have made possible unambiguous rotational assignments, yielding accurate values for the adiabatic ionization energy of CoC(X(2)Σ(+)), IE(CoC) = 62,384.3 ± 0.6 cm(-1) (7.73467 ± 0.00007 eV), the vibrational frequency ωe (+) = 985.6 ± 0.6 cm(-1), the anharmonicity constant ωe (+)χe (+) = 6.3 ± 0.6 cm(-1), the rotational constants (Be (+) = 0.7196 ± 0.0005 cm(-1), αe (+) = 0.0056 ± 0.0008 cm(-1)), and the equilibrium bond length re (+) = 1.534 Å for CoC(+)(X(1)Σ(+)). The observation of the N(+) = 0 level in the PFI-PE measurement indicates that the CoC(+) ground state is of (1)Σ(+) symmetry. Large ΔN(+) = N(+) - N' changes up to 6 are observed for the photoionization transitions CoC(+)(X(1)Σ(+); v(+) = 0-2; N(+)) ← CoC∗((2)Σ(+); v'; N' = 6, 7, 8, and 9). The highly precise energetic and spectroscopic data obtained in the present study have served as a benchmark for testing theoretical predictions based on state-of-the-art ab initio quantum calculations at the CCSDTQ∕CBS level of theory as presented in the companion article.

Rovibrationally selected and resolved pulsed field ionization-photoelectron study of propyne: Ionization energy and spin-orbit interaction in propyne cation

By using a high-resolution infrared (IR) laser to prepare propyne (C(3)H(4)) in selected rotational levels of the excited nu(1) (acetylenic C-H stretching) vibration mode prior to vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) measurements, we have obtained rotationally resolved VUV-PFI-PE spectra for the C(3)H(4) (+)(X (2)E(32,12),nu(1) (+)=1) band. The analysis of these PFI-PE spectra leads to the determination of the spin-orbit constant of A=-13.0+/-0.2 cm(-1) for the C(3)H(4) (+)(X (2)E(32,12),nu(1) (+)=1) state. Using this A constant and the relative rotationally selected and resolved state-to-state photoionization cross sections thus measured, we have obtained an excellent simulation for the VUV-PFI-PE origin band of C(3)H(4) (+)(X (2)E(32,12)), yielding a value of 83 619.0+/-1.0 cm(-1) (10.367 44+/-0.000 12 eV) for the adiabatic ionization energy of C(3)H(4) [IE(C(3)H(4))]. The present two-color IR-VUV-PFI-PE study has also made possible the determination of the C-H stretching frequencies nu(1) (+)=3217.1+/-0.2 cm(-1) for C(3)H(4) (+)(X (2)E(32,12)). The spectral assignment and simulation were guided by high-level ab initio calculations on the IE(C(3)H(4)), Franck-Condon factors for photoionization transitions, and rotational constants and vibrational frequencies for C(3)H(4) (+).

Combined vacuum ultraviolet laser and synchrotron pulsed field ionization study of CH2BrCl

The pulsed field ionization-photoelectron (PFI-PE) spectrum of bromochloromethane (CH2BrCl) in the region of 85,320-88,200 cm-1 has been measured using vacuum ultraviolet laser. The vibrational structure resolved in the PFI-PE spectrum was assigned based on ab initio quantum chemical calculations and Franck-Condon factor predictions. At energies 0-1400 cm-1 above the adiabatic ionization energy (IE) of CH2BrCl, the Br-C-Cl bending vibration progression (nu1+=0-8) of CH2BrCl+ is well resolved and constitutes the major structure in the PFI-PE spectrum, whereas the spectrum at energies 1400-2600 cm-1 above the IE(CH2BrCl) is found to exhibit complex vibrational features, suggesting perturbation by the low lying excited CH2BrCl+(A 2A") state. The assignment of the PFI-PE vibrational bands gives the IE(CH2BrCl)=85,612.4+/-2.0 cm-1 (10.6146+/-0.0003 eV) and the bending frequencies nu1+(a1')=209.7+/-2.0 cm-1 for CH2BrCl+(X2A'). We have also examined the dissociative photoionization process, CH2BrCl+hnu-->CH2Cl++Br+e-, in the energy range of 11.36-11.57 eV using the synchrotron based PFI-PE-photoion coincidence method, yielding the 0 K threshold or appearance energy AE(CH2Cl+)=11.509+/-0.002 eV. Combining the 0 K AE(CH2Cl+) and IE(CH2BrCl) values obtained in this study, together with the known IE(CH2Cl), we have determined the 0 K bond dissociation energies (D0) for CH2Cl+-Br (0.894+/-0.002 eV) and CH2Cl-Br (2.76+/-0.01 eV). We have also performed CCSD(T, full)/complete basis set (CBS) calculations with high-level corrections for the predictions of the IE(CH2BrCl), AE(CH2Cl+), IE(CH2Cl), D0(CH2Cl+-Br), and D0(CH2Cl-Br). The comparison between the theoretical predictions and experimental determinations indicates that the CCSD(T, full)/CBS calculations with high-level corrections are highly reliable with estimated error limits of <17 meV.

A Combined Vacuum Ultraviolet Laser and Synchrotron Pulsed Field Ionization Study of BCl3

AbstractFor Abstract see ChemInform Abstract in Full Text.

Vacuum ultraviolet pulsed field ionization study of ND3: accurate thermochemistry for the ND2-ND2+ and ND3-ND3+ system.

The dissociation of energy-selected ND(3) (+) to form ND(2) (+)+D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method. The breakdown curves for ND(3) (+) and ND(2) (+) give a value of 15.891+/-0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND(2) (+) from ND(3). We have also measured the PFI-PE vibrational bands for ND(3) (+)(X;v(2) (+)=0, 1, 2, and 3), revealing partially resolved rotational structures. The simulation of these bands yields precise ionization energies (IEs) for ND(3) (+) X(0,v(2) (+)=0-3,0,0)<--ND(3) X(0,0,0,0). Using the 0 K AE (ND(2) (+)) and IE(ND(3))=10.200+/-0.001 eV determined in the present study, together with the known 0 K bond dissociation energy for ND(3) [D(0)(D-ND(2))=4.7126+/-0.0025 eV], we have determined the D(0)(ND(2) (+)-D), IE(ND(2)), and 0 K heat of formation for ND(2) (+) to be 5.691+/-0.001 eV, 11.1784+/-0.0025 eV, and 1261.82+/-0.4 kJ/mol, respectively. The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND(2) (+)), indicating that the dissociation of excited ND(3) (+) at energies slightly above the dissociation threshold is prompt, occurring in the time scale </=10(-7) s, as observed for the NH(3) system. The available energetic data for the NH(2)-NH(2) (+) and NH(3)-NH(3) (+) system are found to be in excellent accord with those for the ND(2)-ND(2) (+) and ND(3)-ND(3) (+) system after taking into account the zero-point vibrational energy corrections. This finding indicates that the thermochemical data for these two systems are reliable with well-founded error limits.

Vacuum ultraviolet pulsed-field ionization-photoelectron study of H2S in the energy range of 10-17 eV.

Vacuum ultraviolet pulsed-field ionization-photoelectron (PFI-PE) spectra of H(2)S have been recorded at PFI-PE resolutions of 0.6-1.0 meV in the energy range of 10-17 eV using high-resolution synchrotron radiation. The PFI-PE spectrum, which covers the formation of the valence electronic states H(2)S(+) (X (2)B(1), A (2)A(1), and B (2)B(2)), is compared to the recent high-resolution He I photoelectron spectra of H(2)S obtained by Baltzer et al. [Chem. Phys. 195, 403 (1995)]. In addition to the overwhelmingly dominated origin vibrational band, the PFI-PE spectrum for H(2)S(+)(X (2)B(1)) is found to exhibit weak vibrational progressions due to excitation of the combination bands in the nu(1) (+) symmetric stretching and nu(2) (+) bending modes. While the ionization energy (IE) for H(2)S(+)(X (2)B(1)) obtained here is in accord with values determined in previously laser PFI-PE measurements, the observation of a new PFI-PE band at 12.642+/-0.001 eV suggests that the IE for H(2)S(+)(A (2)A(1)) may be 0.12 eV lower than that reported in the He I study. The simulation of rotational structures resolved in PFI-PE bands shows that the formation of H(2)S(+)(X (2)B(1)) and H(2)S(+)(A (2)A(1)) from photoionization of H(2)S(X (1)A(1)) is dominated by type-C and type-B transitions, respectively. This observation is consistent with predictions of the multichannel quantum defect theory. The small changes in rotational angular momentum observed are consistent with the dominant atomiclike character of the 2b(1) and 5a(1) molecular orbitals of H(2)S. The PFI-PE measurement has revealed perturbations of the (0, 6, 0) K(+)=3 and (0, 6, 0) K(+)=4 bands of H(2)S(+)(A (2)A(1)). Interpreting that these perturbations arise from Renner-Teller interactions at energies close to the common barriers to linearity of the H(2)S(+) (X (2)B(1) and A (2)A(1)) states, we have deduced a barrier of 23,209 cm(-1) for H(2)S(+)(X (2)B(1)) and 5668 cm(-1) for H(2)S(+)(A (2)A(1)). The barrier of 23 209 cm(-1) for H(2)S(+)(X (2)B(1)) is found to be in excellent agreement with the results of previous studies. The vibrational PFI-PE bands for H(2)S(+)(B (2)B(2)) are broad, indicative of the predissociative nature of this state.

Vacuum ultraviolet laser pulsed field ionization photoelectron study of trans-2-butene

The single-photon pulsed field ionization photoelectron (PFI-PE) spectrum of trans-2-butene (trans-CH3CH=CHCH3) in the energy range of 73 500–75 850 cm−1 has been measured using vacuum ultraviolet laser sources. The semi-empirical simulation of fine structures resolved in the original PFI-PE band yields a value of 73 624.7±2.0 cm−1 for the ionization energy (IE) of trans-2-butene. The vibrational bands for trans-CH3CH=CHCH3+ resolved in the PFI-PE spectrum are assigned based on ab initio calculations of the vibrational frequencies and Franck-Condon factors (FCFs) for ionization transitions. This assignment has provided reliable vibrational frequencies (ν1+=104 cm−1, ν2+=127 cm−1, ν3+=131 cm−1, ν5+=484 cm−1, ν8+=798 cm−1, ν13+=1164 cm−1, ν14+=1264 cm−1, ν16+=1307 cm−1, ν20+=1407 cm−1, and ν22+=1567 cm−1) for trans-CH3CH=CHCH3+. The PFI-PE spectrum is compared to the recently reported PFI-photoion (PFI-PI) spectrum for trans-2-butene. The major difference observed between the PFI-PE and PFI-PI spectra is that the intensities for excited vibrational bands were significantly suppressed or indiscernible in the PFI-PI spectrum, suggesting that the lifetimes for high-n Rydberg states associated with these excited vibrational bands were greatly reduced under the conditions used in the PFI-PI study. The experimental conditions used in the PFI-PI study also led to an IE value of about 20 cm−1 lower than that obtained in the PFI-PE measurement. We have also reassigned the vibrational bands resolved in the PFI-PE spectrum for cis-2-butene based on the FCF calculation and a more reliable set of theoretical vibrational frequencies.

Pulsed field ionization-photoelectron bands for CS2+ in the energy range of 13.2–17.6 eV: An experimental and theoretical study

Vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for CS2 have been obtained in the energy range of 13.2–17.6 eV, revealing complex vibronic structures for the CS2+(B̃ 2Σu+ and C̃ 2Σg+) states. The PFI-PE spectra for CS2+(B̃ 2Σu+ and C̃ 2Σg+) are dominated by the ν1+ (symmetric stretching) mode. However, PFI-PE bands due to the ν2+ (bending) and ν3+ (antisymmetric stretching) modes with both even and odd quanta are clearly resolved. The simulation of rotational contours resolved in the origin PFI-PE bands yields accurate ionization energies of 14.4742±0.0005 eV (116 742±4 cm−1) and 16.1883±0.0005 eV (130 567±4 cm−1) for the formation of CS2+(B̃ 2Σu+ and C̃ 2Σg+) states from CS2(X̃ 1Σg+), respectively. The PFI-PE bands for Σu+2(302) and Σg+2(303) at 14.805 and 14.965 eV, which are in near energy resonance with the 0 K dissociation thresholds for the formation of S+(4S)+CS(X 1Σ+; ν″=0 and 1) from CS2, respectively, are found to be enhanced. These enhancements are rationalized by the prompt dissociation of excited CS2 in high-n (n⩾100) Rydberg states prior to PFI. Three-dimensional potential energy functions (PEFs) for CS2+(2 2Πu, B̃ 2Σu+, and C̃ 2Σg+) states have been generated theoretically using the complete active space self-consistent field and internally contracted multireference configuration interaction methods. On the basis of these PEFs, the harmonic frequencies for CS2+(2 2Πu, B̃ 2Σu+, and C̃ 2Σg+) and vibronic energy levels for CS2+(B̃ 2Σu+) have been calculated variationally. These theoretical predictions have made possible a satisfactory assignment of the vibronic bands resolved in the PFI-PE spectra for CS2+(B̃ 2Σu+ and C̃ 2Σg+). Using the theoretical predictions obtained here, we have also assigned vibronic bands for CS2+(2 2Πu) observed in the HeI photoelectron spectrum [Baltzer et al., Chem. Phys. 202, 185 (1996)].

Unimolecular decay pathways of state-selected CO2+ in the internal energy range of 5.2–6.2 eV: An experimental and theoretical study

The vacuum ultraviolet pulsed field ionization (PFI)-photoelectron (PFI-PE) spectrum of CO2 has been measured in the energy region of 19.0–20.0 eV. The PFI-PE vibrational bands resolved for CO2+(C 2Σg+) are overwhelmingly dominated by the origin band along with weak vibrational bands corresponding to excitations of the ν1+ (symmetric stretching), ν2+ (bending), and ν3+ (antisymmetric stretching) modes. The simulation of the rotational contour resolved in the origin PFI-PE band yields a value of 19.3911±0.0005 eV for the ionization energy of CO2 to form CO2+(C 2Σg+). A PFI-PE peak is found to coincide with each of the 0 K dissociation thresholds for the formation of O+(4S)+CO(X 1Σ+) and CO+(X 2Σ+)+O(3P). This observation is tentatively interpreted to result from the lifetime switching effect, arising from the prompt dissociation of excited CO2 in high-n (n⩾100) Rydberg states prior to PFI. We have also examined the decay pathways for state-selected CO2+ in the internal energy range of 5.2–6.2 eV using the PFI-PE-photoion coincidence scheme. The coincidence TOF data show unambiguously the formation of O+(4S)+CO(X 1Σ+;ν″=0,1) and CO+(X 2Σ+;ν+=0,1)+O(3P). Analysis of the kinetic energy releases of fragment ions suggests that the dissociation of excited CO2+ involved is nonstatistical and proceeds with an impulsive mechanism. Potential energy functions (PEFs) for the CO2+(C 2Σg+) state and the lowest quartet states of CO2+, together with their spin–orbit interactions, have been calculated using the complete active space self-consistent field and internal contracted multireference configuration interaction methods. Based on these PEFs, vibrational levels for CO2+(C 2Σg+) have been also calculated using a variational approach. With the aid of these theoretical calculations, vibrational bands resolved in the PFI-PE spectrum for CO2+(C 2Σg+) have been satisfactorily assigned, yielding a ν3+ value of 2997 cm−1. The theoretical calculation also provides a rationalization that the predissociation for CO2+(C 2Σg+) to form O+(4S)+CO(X 1Σ+) and CO+(X 2Σ+)+O(3P) most likely proceeds via the repulsive a 4Σg− and b 4Πu (or B14 in a bent geometry) states.

Vacuum ultraviolet pulsed field ionization-photoelectron study of OCS in the energy range of 15–19 eV

Vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for OCS have been obtained in the energy range 15.0–19.0 eV, covering the vibronic bands of OCS+ (A 2Π, B 2Σ+, and C 2Σ+). The ionization energies for the formation of the ground vibrational levels of OCS+ (A 2Π3/2, A 2Π1/2, B 2Σ+, and C 2Σ+) from the ground OCS(X 1Σ+) state have been determined as 15.0759±0.0005 eV, 15.0901±0.0005 eV, 16.0403±0.0005 eV, and 17.9552±0.0005 eV, respectively. We have also generated the theoretical adiabatic three dimensional potential energy functions (PEFs) for OCS+(A 2Π) by employing the complete active space self-consistent field and internally contracted multireference configuration interaction methods. Using these PEFs, the spectroscopic constants and low-lying rovibronic energy levels for OCS+(A 2Π) are calculated variationally. These calculations have made possible the identification of many PFI-PE vibronic bands for OCS+(A 2Π), which are originated from vibronic and Fermi resonance interactions. Owing to the different equilibrium geometries between the OCS+(A 2Π) and OCS(X 1Σ+) states, the PFI-PE spectrum for OCS+(A 2Π) exhibits a long vibronic progression extending well above the OCS+(B 2Σ+) state. On the contrary, the PFI-PE spectra for OCS+ (B 2Σ+ and C 2Σ+) are overwhelmingly dominated by the ground (0,0,0) bands, exhibiting only weak vibrational progressions.

A pulsed field ionization study of the dissociative photoionization reaction [formula omitted

We have examined the titled dissociation reaction near its 0 K threshold using the pulsed field ionization-photoelectron (PFI-PE) and PFI-PE-photoion coincidence methods. This study yields a value of 14.4184±0.0010 eV for the 0 K threshold. The PFI-PE spectrum for CD 4 exhibits a sharp step at the 0 K threshold, indicating that the dissociation of excited CD 4 in high- n ( n⩾100) Rydberg states at energies above the dissociation threshold occurs in ⩽10 −7 s. The energetic data for CD 3/ CD 3 + and CD 4/ CD 4 + are found to be in excellent agreement with those for CH 3/ CH 3 + and CH 4/ CH 4 + after taking into account the zero-point-vibrational-energy corrections.

High-resolution energy-selected study of the reaction CH3X+→CH3++X: Accurate thermochemistry for the CH3X/CH3X+ (X=Br, I) system

Using the high-resolution pulsed field ionization-photoelectron (PFI-PE) and PFI-PE-photoion coincidence (PFI-PEPICO) techniques, we have examined the formation of methyl cation (CH3+) from the dissociation of energy-selected CH3X+ (X=Br and I) near their dissociation thresholds. The breakdown diagrams for CH3X thus obtained yield values of 12.834±0.002 eV and 12.269±0.003 eV for the 0 K dissociative threshold or appearance energy (AE) for CH3+ from CH3Br and CH3I, respectively. Similar to the observation in PFI-PE studies of CH4, C2H2, and NH3, the PFI-PE spectrum for CH3Br exhibits a step at the 0 K AE for CH3+, indicating that the dissociation of excited CH3Br in high-n (⩾100) Rydberg states at energies slightly above the dissociation threshold occurs in a time scale of ⩽10−7 s. The observed step is a confirmation of the 0 K AE(CH3+) from CH3Br determined in the PFI-PEPICO study. The adiabatic ionization energies (IEs) for the CH3Br+(X̃ 2E3/2,1/2) spin–orbit states were determined by PFI-PE measurements to be 10.5427±0.0010 and 10.8615±0.0010 eV, respectively, yielding the spin–orbit coupling constant to be 2571±4 cm−1. The AE(CH3+) values from CH3Br and CH3I and the IE[CH3Br+(X̃ 2E3/2)] value obtained here, when combined with the known IE of CH3 (9.8380±0.0004 eV) and IE[CH3I+(X̃ 2E3/2)] (9.5381±0.0001 eV), have allowed accurate determination of the 0 K bond dissociation energies for CH3–Br (2.996±0.002 eV), CH3+–Br (2.291±0.002 eV), CH3–I (2.431±0.003 eV), and CH3+–I (2.731±0.003 eV). Using the AE(CH3+) from CH3Br and CH3I, together with the known 0 K heats of formation (ΔfH00) for Br (117.93±0.13 kJ/mol), I (107.16±0.04 kJ/mol), and CH3+ (1099.05±0.33 kJ/mol), we have obtained more precise ΔfH00 values for CH3Br (−21.30±0.42 kJ/mol) and CH3I (22.43±0.50 kJ/mol). This experiment demonstrated that highly reliable ΔfH00 values for a range of molecules with error limits comparable to those for some of the most precisely measured values, such as ΔfH00(CH4), can be obtained by PFI-PE and PFI-PEPICO measurements.

High-resolution energy-selected study of the reaction NH3+→NH2++H: Accurate thermochemistry for the NH2/NH2+ and NH3/NH3+ systems

Employing the newly developed high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, we have examined the dissociation of energy-selected NH3+ to form NH2++H near its threshold. The breakdown curves for NH2+ and NH3+ thus obtained yield a value of 15.765±0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for NH2+ from NH3. This value, together with the known ionization energy (IE=10.1864±0.0001 eV) and 0 K bond dissociation energy (D0=4.6017±0.0025 eV) for NH3, allows the determination of the D0(NH2+–H) and IE(NH2), which are 5.5786±0.0010 and 11.1633±0.0025 eV, respectively. Using the known 0 K heats of formation (ΔHf0°) for NH3 and H and the AE(NH2+), we obtain the ΔHf0°(NH2+)=302.60±0.08 kcal/mol. The PFI-PE spectrum for NH3 exhibits a step-like feature at the 0 K AE(NH2+), indicating that the dissociation of excited NH3 in high-n (n⩾100) Rydberg states at energies slightly above the dissociation threshold occurs on a time scale ⩽10−7 s. This step confirms the AE(NH2+) value derived from the PFI-PEPICO measurements. Highly accurate energetic data with well-founded error limits, such as those obtained in the present and other studies using the PFI techniques, are expected to play an important role for the development of the next generation of ab initio quantum computation procedures. This experiment has stimulated a state-of-the-art ab initio quantum chemical calculation [Dixon et al., J. Chem. Phys. 115, 2576 (2001)]. The comparison between theoretical predictions and the best experimental results for the NH2/NH2+ and NH3/NH3+ systems indicates that the accuracy of the computational scheme used is ⩽0.4 kcal/mol.

Rotationally resolved pulsed-field ionization photoelectron bands for O2+(A 2Πu,v+=0–12) in the energy range of 17.0–18.2 eV

We have obtained rotationally resolved pulsed-field ionization photoelectron (PFI-PE) spectra for O2 in the energy range of 17.05–18.13 eV, covering the ionization transitions O2+(A 2Πu,v+=0–12,N+)←O2(X 3Σg−,v″=0,N″). Although these O2+(A 2Πu,v+) PFI-PE bands have significant overlaps with vibrational bands for O2+(a 4Πu) and O2+(X 2Πg), we have identified all the O2+(A 2Πu,v+=0–12) bands by simulation of spectra obtained using supersonically cooled O2 samples with rotational temperatures ≈20 and 220 K. While these v+=0–12 PFI-PE bands represent the first rotationally resolved photoelectron data for O2+(A 2Πu), the PFI-PE bands for O2+(A 2Πu,v+=9 and 10) are the first rotationally resolved spectroscopic data for these levels. The simulation also allows the determination of accurate ionization energies, vibrational constants, and rotational constants for O2+(A 2Πu,v+=0–12). The analysis of the PFI-PE spectra supports the conclusion of the previous emission study that the O2+(A 2Πu,v+=9 and 10) states are strongly perturbed by a nearby electronic state.

Pulsed field ionization-photoelectron photoion coincidence spectroscopy with synchrotron radiation: the heat of formation of the C2H5+ ion

Pulsed field ionization photoelectron (PFI-PE) spectroscopy combined with ion coincidence detection has been used with multi-bunch synchrotron radiation at the Advance Light Source (ALS) to energy select ions and to measure their breakdown diagram. The resolution for ion state selection achieved with Ar+ (2P3/2, 1/2) employing this PFI-PE-photoion coincidence apparatus is 0.6 meV (full width at half maximum). The production of C2H5+ from C2H5Br was investigated near the dissociative photoionization limit with this pulsed field ionization-threshold photoelectron photoion coincidence (PFI-PEPICO) scheme. Although the PFI-PE spectra of C2H5Br, C2H5I, and benzene show that the production of ions in the Franck-Condon gap regions is quite low, the selectivity for PFI-PE detection and the suppression of prompt electrons is such that we can detect 1 PFI-PE out of 25,000 total electrons s-1. The derived C2H5+ heat of formation from the analysis of the C2H5Br+ breakdown diagram and a critical analysis of other results is 900.5 +/- 2.0 kJ mol-1 at 298 K, or 913.2 +/- 2.0 kJ mol-1 at 0 K. This leads to an ethylene proton affinity at 298 K of 682.0 kJ mol-1. The measured IE of C2H5Br is 10.307 eV.

Rotationally resolved pulsed field ionization photoelectron study of CO+(X 2Σ+,v+=0–42) in the energy range of 13.98–21.92 eV

We have obtained rotationally resolved pulsed field ionization–photoelectron (PFI-PE) spectra of CO in the energy range of 13.98–21.92 eV, covering the ionization transitions CO+(X 2Σ+,v+=0–42,N+)←CO(X 1Σ+,v″=0,N″). The PFI-PE bands for CO+ (X 2Σ+, v+=8–22, 24, and 28–39) obtained here represent the first rotationally resolved spectroscopic data for these states. The high-resolution features observed in the PFI-PE spectra allow the identification of vibrational bands for the CO+ (X 2Σ+, v+=10, 14, 15, 17, 18, 21, 24, 25, 29–31, 33, 35–37, and 39) states, which strongly overlap with prominent vibrational bands of the CO+(A 2Π3/2,1/2,B 2Σ+) states. The simulation using the Buckingham–Orr–Sichel model has provided accurate molecular constants for CO+(X 2Σ+,v+=0–42), including ionization energies, vibrational constants (ωe+=2218.8±3.5 cm−1, ωe+xe+=16.20±0.32 cm−1, ωe+ye+=0.074±0.011 cm−1, and ωe+ze+=−0.001 83±0.000 13 cm−1), and rotational constants [Be+=1.9797±0.0051 cm−1, αe+=0.0201±0.0011 cm−1, γe+=0.000 122±0.000 067 cm−1, ze+=−(5.2±1.1)×10−6 cm−1]. Enhancement of ΔN&amp;lt;0 rotational branches, attributable to field-induced rotational autoionization, was clearly discernible in PFI-PE bands for CO+ (X 2Σ+, v+=0–5, 11, and 12). Significant local enhancements due to near-resonance autoionization were observed for low v+ (&amp;lt;10) PFI-PE bands of CO+(X 2Σ+), where the density of interloper Rydberg states converging to higher ionic levels is high as manifested in the photoion spectrum. The observation of a long vibrational progression in the Franck–Condon gap region, where strong autoionization states are absent, is consistent with the suggestion that high-n Rydberg states converging to highly excited vibrational levels of CO+(X 2Σ+) are partially populated via direct excitation to a repulsive neutral state. The relatively minor band intensity variation observed for high v+ PFI-PE bands is also in accord with the direct excitation model. Since ΔN=0, ±1, ±2, and ±3 rotational branches are observed in the PFI-PE spectra, we conclude that the ejected photoelectrons are restricted to angular momentum continuum states l=0–4.

A high resolution pulsed field ionization photoelectron study of O2 using third generation undulator synchrotron radiation

We have improved a newly developed experimental scheme for high resolution pulsed field ionization photoelectron (PFI-PE) studies [Hsu et al., Rev. Sci. Instrum. (in press)] using the high resolution monochromatized multibunch undulator synchrotron source of the Chemical Dynamics Beamline at the Advanced Light Source. This improved scheme makes possible PFI-PE measurements with essentially no contamination by background electrons arising from direct photoionization and prompt autoionization processes. We present here a preliminary analysis of the rotationally resolved PFI-PE spectrum for O2 obtained at a resolution of 0.5 meV (full-width-at-half-maximum) in the photon energy range of 18.1–19.4 eV, yielding accurate ionization energies for the transitions O2+(b 4Σg−, v+=0–9, N+=1)←O2(X 3Σg−, v=0, N=1).