Pulsed Field Ionization Photoelectron Research Articles

A high-resolution pulsed field ionization-photoelectron-photoion coincidence study of vinyl bromide.

By employing the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method, we have examined the unimolecular dissociation reaction of energy-selected C(2)H(3)Br(+) to form C(2)H(3) (+)+Br near its threshold. The analysis of the breakdown curves for C(2)H(3)Br(+) and C(2)H(3) (+) yields a value of 11.9010+/-0.0015 eV for the 0 K dissociative photoionization threshold or appearance energy (AE) for C(2)H(3) (+) from C(2)H(3)Br. This AE(C(2)H(3) (+)) value, together with the ionization energy (IE) for C(2)H(3)Br (9.8200+/-0.0015 eV) obtained by PFI-PE and threshold photoelectron (TPE) measurements, has allowed the determination of the 0 K dissociation energy (D(0)) for the C(2)H(3) (+)-Br bond to be 2.081+/-0.002 eV. The 0 K AE(C(2)H(3) (+)) from C(2)H(3)Br obtained in this study corresponds to DeltaH(f0) ( composite function )(C(2)H(3) (+))=1123.7+/-1.9 kJ/mol. Combining the latter value and the known DeltaH(f0) ( composite function )(C(2)H(3))=306.7+/-2.1 kJ/mol, we calculated a value of 8.468+/-0.029 eV for the IE(C(2)H(3)), which is in accord with the result obtained in the previous photoionization efficiency study. We have also carried out high-level ab initio calculations for the IE(C(2)H(3)) at the Gaussian-3 and the CCSD(T,full)/CBS level of theory. The CCSD(T,full)/CBS prediction of 8.487 eV for the IE(C(2)H(3)-->bridged-C(2)H(3) (+)) is in good agreement with the IE(C(2)H(3)) value derived in the present experiment. Combining the 0 K AE(C(2)H(3) (+))=11.9010+/-0.0015 eV and the IE(C(2)H(3))=8.468+/-0.029 eV yields the value of 3.433+/-0.029 eV for D(0)(C(2)H(3)-Br). We have also recorded the TPE spectrum of C(2)H(3)Br in the energy range of 9.80-12.20 eV. Members (n=5-14) of four autoionizing Rydberg series converging to the C(2)H(3)Br(+)(A (2)A(')) state are observed in the TPE spectrum. The analysis of the converging limit of these Rydberg series and the vibrational TPE bands for C(2)H(3)Br(+)(A (2)A(')) has provided more precise values for the nu(6) (+) (1217+/-10 cm(-1)) and nu(8) (+) (478+/-8 cm(-1)) modes and the IE (10.9156+/-0.0010 eV) for the formation of C(2)H(3)Br(+)(A (2)A(')) from C(2)H(3)Br.

Single-photon vacuum-ultraviolet laser-pulsed-field ionization-photoelectron studies of trans- and cis-1-bromopropenes.

The vacuum-ultraviolet (VUV) pulsed-field ionization-photoelectron (VUV-PFI-PE) spectra of trans-1-bromopropene (trans-CH(3)CH[Double Bond]CHBr) and cis-1-bromopropene (cis-CH(3)CH[Double Bond]CHBr) have been measured in the energy region of 74 720-76 840 cm(-1). The simulation of fine structures observed in the origin VUV-PFI-PE vibrational bands of these molecules has provided the ionization energies (IEs) of trans-1-bromopropene and cis-1-bromopropene to be 74 779.3+/-2.0 cm(-1) (9.2715+/-0.0002 eV) and 75 140.2+/-2.0 cm(-1) (9.3162+/-0.0002 eV), respectively. The vibrational bands resolved in these VUV-PFI-PE spectra at energies 0-1700 cm(-1) above the IEs of trans-1-bromopropene and cis-1-bromopropene have been assigned based on theoretical vibrational frequencies and calculated Franck-Condon factors for the ionization transitions.

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.

A state-selected study of the H2+(X,v+=0–17,N+=1)+Ne proton transfer reaction using the pulsed-field ionization–photoelectron–secondary ion coincidence scheme

The endothermic proton transfer reaction, H2+(v+,N+=1)+Ne→NeH++H(ΔH=0.54 eV), is investigated over a broad range of reactant vibrational energies using the pulsed-field ionization–photoelectron–secondary ion coincidence (PFI–PESICO) scheme. For the lowest vibrational levels, v+=0 and 1, a detailed translational energy dependence is also presented using a continuous approach for preparing reactant ions with monochromatic VUV. Sharp threshold onsets are observed, suggesting the importance of long-lived intermediates or resonances. At a translational energy, ET=0.7 eV, absolute state-selected reaction cross sections are measured for all reactant vibrational levels v+=0–17. For levels v+=0–6, the cross sections grow rapidly with vibrational quantum, above which the cross sections saturate at a value of ∼13±4 Å2. At levels v+&amp;gt;13, the cross sections decline, probably due to competition with the dissociation channel. At a translational energy, ET=1.7 eV, absolute state-selected reaction cross sections are measured for reactant vibrational levels spanning the range between v+=0 and 14. Cross section growth is observed from v+=0–7, above which the cross sections no longer exhibit a steady trend. At ET=4.5 eV, cross sections are reported for vibrational levels covering the range between v+=0 and 12. The cross sections are substantially lower at this high translational energy, however, they still exhibit a substantial vibrational enhancement below v+=8. The present measurements are compared with quasiclassical trajectory (QCT) calculations. The comparison can be categorized by three distinct total energy (Etot=ET+Evib) regimes. For Etot&amp;lt;1 eV, the experimental cross sections exceed the QCT results, consistent with important quantum effects at low energies. For 1&amp;lt;Etot&amp;lt;3 eV, excellent agreement is observed between the PFI–PESICO cross sections and the QCT calculations. At total energies exceeding 3 eV, the experimental results are generally higher, probably because QCT overpredicts competition from the dissociation channel.

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.

High-resolution state-selected ion-molecule reaction studies using pulsed field ionization photoelectron-secondary ion coincidence method

We have developed an octopole-quadrupole photoionization apparatus at the Advanced Light Source for absolute integral cross-section measurements of rovibrational-state-selected ion-molecule reactions. This apparatus consists of a high-resolution photoionization ion source, a wired ion gate lens, a dual radio-frequency (rf) octopole ion guide reaction gas cell, and a quadrupole mass spectrometer for reactant and product ion detection. The unique feature of this apparatus is the implementation of the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, which has allowed the rotational-state selection of diatomic ions for ion-molecule reaction studies. The novel application of the wired ion gate lens for the rejection of false coincidence background ions is described. This application, along with the differential-ion-gate scheme, has made possible the measurements of rovibrational-state-selected absolute integral reaction cross sections for ion-molecule collisions using the PFI-PE-secondary ion coincidence (PFI-PESICO) method. The successful measurement of absolute state-selected cross sections for H2+(X,v+,N+)+Ar(Ne) with v+ up to 17 [the third to the last vibrational state of H2+(X)] demonstrates the high sensitivity of this differential-ion-gate PFI-PESICO method. In order to gain a detailed understanding and to obtain optimal performance of the wired ion gate lens for PFI-PESICO measurements, we have carried out ion trajectory calculations of reactant ions between the photoionization region and the rf-octopole ion guide. On the basis of these calculations, possible future improvements for the application of this differential-ion-gate PFI-PESICO scheme are discussed.

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 laser pulsed field ionization photoelectron study of cis-2-butene

The vacuum ultraviolet pulsed field ionization-photoelectron spectra of supersonically cooled cis-2-butene (cis-CH3CH=CHCH3) have been measured in the photon energy range of 73 560–75 460 cm−1. Using the ab initio theoretical rotational constants of cis-CH3CH=CHCH3 and its cation (cis-CH3CH=CHCH3+) and a semiempirical simulation scheme, we have obtained a good fit of the origin vibrational band with partially resolved contours of rotational branches. After taking into account the Stark shift, the ionization energy of cis-CH3CH=CHCH3 is determined to be 73 595.0±1.5 cm−1. Guided by ab initio vibrational frequency calculations, we have also assigned the vibrational bands observed for cis-CH3CH=CHCH3+ in its ground state.

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 photoelectron–photoion coincidence study of the dissociative photoionization process D 2O+ hν→OD ++D+e −

We have examined the titled reaction near its threshold using the pulsed field ionization photoelectron–photoion coincidence method. The measured 0 K threshold (18.220±0.002 eV) has made possible the determination of more precise values for the 0 K bond dissociation energies for D–OD + (5.584±0.002 eV) and D–OD (5.191±0.002 eV) and the 0 K heats of formation ( ΔH 0 f0's) for OD + (308.83±0.05 kcal/mol) and OD (8.38±0.05 kcal/mol). We found that the available energetic data for the OD/OD + and D 2O/D 2O + system are in excellent agreement with those for the OH/OH + and H 2O/H 2O + system after taking into account the zero-point vibration energies of these species.

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.

Observation of accurate ion dissociation thresholds in pulsed field ionization-photoelectron studies.

We report the first observation, together with a mechanism for such an observation, of a steplike feature in the pulsed field ionization photoelectron measurement of CH4(C2H2), marking the 0 K dissociation threshold for the formation of CH3(+) + H(C2H(+) + H) from CH4(C2H2). The nonexistence of a step in the spectrum for C 2H4 at its dissociation threshold for C2H2(+) formation provides strong support for the proposed mechanism. This experiment shows that, for a range of molecules, where the ion dissociation lifetimes near the dissociation thresholds are <10(-7) s, pulsed field ionization photoelectron measurements will yield not only highly accurate ionization energies, but also 0 K dissociation thresholds.

Vacuum ultraviolet photoionization and photoelectron studies in the new millennium: recent developments and applications

Vacuum ultraviolet (VUV) photoionization mass spectrometry and photoelectron spectroscopy have played a central role in providing energetic and spectroscopic information for neutrals and cations. The most important data obtainable in a VUV photoionization and photoelectron experiment are ionization energies and 0 K ion dissociation thresholds or appearance energy (AE), from which 0 K bond dissociation energies for neutrals and cations can be deduced. The recent developments in VUV lasers and third-generation synchrotron sources, together with the introduction of the pulsed-field ionization (PFI), photoelectron (PFI-PE), and PFI-photoion (PFI-PI) methods, have revolutionized the field of photoelectron and ion spectroscopy by significantly improving the energy resolution to the range of 0.025–1.0 meV (full width at half maximum, FWHM). These resolutions, which make possible the measurement of photoelectron spectra for many simple molecules at the rotational-resolved level, are ≈100-fold better than those observed in traditional photoelectron studies, making the PFI-PE technique a true spectroscopic method. The recent introduction of the synchrotron-based PFI-PEPICO scheme has shown that AE values for a range of molecules can be determined with an unprecedented precision limited only by the PFI-PE measurement. The synchrotron-based PFI-PEPICO and PFI-PI schemes show great promises for future studies of state- or energy-selected ion-dissociation dynamics and energy-selected ion-molecule reaction dynamics. Further improvement in energy resolution for PFI-PE and PFI-PI measurements has been demonstrated using the two-color photo-induced Rydberg ionization (PIRI) spectroscopic scheme, which involves the photo-induced ionization of intermediate long-lived high- n ( n≥100) Rydberg states. The incorporation of this method by VUV photoexcitation to prepare intermediate high- n ( n≥100) Rydberg states is also expected to greatly increase the energy range of PFI studies. The availability of this array of laser- and synchrotron-based PFI methods, including PFI-PE, PFI-PEPICO, PFI-PI, PFI-ion-pair, and PIRI schemes, ensures an exciting and bright future for VUV photoionization and photoelectron studies in the new millennium.

Pulsed field ionization–photoelectron bands for CO2+(A 2Πu and B 2Σu+) in the energy range of 17.2–19.0 eV: An experimental and theoretical study

The vacuum ultraviolet (VUV) pulsed field ionization–photoelectron (PFI–PE) spectra for CO2 have been measured in the energy range of 17.2–19.0 eV, showing complex vibronic structures for the CO2+(A 2Πu and B 2Σu+) states. The PFI–PE spectra for CO2+(A 2Πu and B 2Σu+) are dominated by the v1+ (symmetric stretching) vibrational progressions, and weak bands due to excitation of both even and odd quanta of the v2+ (bending) and v3+ (antisymmetric stretching) modes are observed in the VUV–PFI–PE spectra. The simulation of rotational contours resolved in the PFI–PE vibronic bands associated with excitation to CO2+(A 2Π3/2,1/2u;v1+=0–5,v2+=0,v3+=0) and CO2+(B 2Σu+;0,0,0) has yielded accurate ionization energies for the formation of these states from CO2(X 1Σg+). Three-dimensional potential energy functions (PEFs) for CO2+(B 2Σu+) have also been generated theoretically using the complete active space self-consistent field and internally contracted multireference configuration interaction methods. Based on these PEFs, vibrational energy levels for CO2+(B 2Σu+), together with the Franck–Condon factors for their formation from CO2(X 1Σg+), have been calculated. With the guide of these theoretical predictions, the vibrational bands resolved in the PFI–PE spectrum for CO2+(B 2Σu+) have been satisfactorily assigned. This assignment reveals the nature of many vibrational PFI–PE bands as originated from anharmonic resonance interactions and members of Fermi polyads.

High-resolution pulsed-field ionization photoelectron–photoion coincidence spectroscopy:

Abstract We have established a high-resolution vacuum ultraviolet (VUV) photoionization–photoelectron facility at the chemical dynamics beamline of the advanced light source. This facility is equipped with a 6.65-m monochromator, which has demonstrated a resolving power of ≈100 000 in the range of 6–30 eV. This resolving power is close to that achieved in common VUV lasers. Taking advantage of this high-resolution capability, we have developed novel synchrotron based pulsed-field ionization–photoelectron (PFI–PE) schemes, attaining resolutions of 1–5 cm−1 (FWHM), which are also similar to that attained in laser-based PFI–PE studies. Most recently, we have introduced a generally applicable PFI–PE–photoion coincidence (PFI–PEPICO) method, making possible the internal state selection for cations to a resolution of 4–5 cm−1 (FWHM) limited only by the PFI–PE measurement. The PFI–PEPICO study of the reaction O2+hν→O+(4S)+O(3P)+e− demonstrates that the dissociation dynamics of diatomic cations can be examined in specific rotational levels using this method. This study also supports that the dissociation threshold for O+(4S)+O(3P) lies at O2+(b4Σg−, v+=4, N+=9). In the PFI–PE–PICO study of CH4 (C2H2), we show that the 0 K dissociative photoionization threshold for CH3+ (C2H+) from CH4 (C2H2) can be determined unambiguously by the disappearance energy of the CH4+ (C2H2+) parent ion to an accuracy of ±0.001 eV. These measurements, when combined with accurate ionization energies for CH3, CH4 and C2H2 obtained by PFI–PE measurements, have allowed the determination of highly accurate 0 K bond dissociation energies for H–CH3, H–CH3+, and H–C2H+. The PFI–PEPICO study of 2-C3H7X (X=Cl, Br and I) has yielded more accurate values for the heat of formation of C3H7+ and the proton affinity of C3H6.

Pulsed-field ionization photoelectron spectrum of the [formula omitted] state of CO +

Calculations of the pulsed-field ionization photoelectron (PFI-PE) spectra of the CO + A 2Π , v +=1 reproduce relatively well the experimental spectra including their intensity anomalies in the ratio of the two subbands, for rotational temperatures of respectively 12 and 298 K.