Photofragment Excitation Spectra Research Articles

Vacuum ultraviolet photoexcitation and photofragment spectroscopic studies of 14N15N between 109000 and 117500cm-1.

In this study, we employed a newly built time-slice velocity-map ion imaging setup, equipped with two tunable vacuum ultraviolet (VUV) laser sources, to obtain the first comprehensive high-resolution photoexcitation and photofragment excitation spectra of 14N15N in the VUV photon energy range 109 000-117 500cm-1. The spectroscopic simulation program PGOPHER was used to analyze the rotationally resolved spectra. Band origins, rotational constants, and isotope shifts compared with those of 14N2 have been obtained for 31 electric-dipole-allowed vibrational states of 14N15N in the aforementioned energy range. These spectroscopic parameters are found to depend on the vibrational quantum number irregularly. Systematic perturbations of the rotational transition energies and predissociation rates within individual absorption bands have also been observed. These are proved to be caused by the strong homogeneous interactions between the valence b'1Σu + state and the Rydberg cn ' 1Σu + states, and between the valence b1Πu states and the Rydberg o3 1Πu states. Heterogeneous interactions between the Rydberg cn 1Πu states and cn ' 1Σu + states also play an important role.

Photodissociation dynamics of carbon dioxide cation via the vibrationally mediated [formula omitted] states in the wavelength range of 282–293 nm

Photodissociation dynamics of CO2+ vibrationally mediated via its A~2Πu,1/2υ1,υ2,0 and B~2Σu+0,0,0 states in the wavelength range of 282–293 nm has been investigated using the time-sliced velocity map imaging technology by probing the CO+. An initial transition from X~2∏g,1/20,0,0 to A~2Πu,1/25,2,0κ followed by strong vibrational coupling between the A~2Πu,1/25,2,0κ and B~2Σu+0,1,0 states is suggested to attribute the origin of the five unassigned peaks in the photofragment excitation spectrum. Since the two-photon energy of 8.46–8.80 eV is close to 8.9 eV, the CO2+C~2Σg+ state is dissociated by the mechanism of Praet et al.

A2Πu and 14Σu- States of Br2+ Studied by [1+1] Two-Photon Dissociation Spectroscopy in a Cold Ion Beam.

The A2Πu-X2Πg and 14Σu--X2Πg electronic transition spectra of Br2+ have been studied in the 500-720 nm wavelength range in a cold ion beam using a cryogenic cylindrical ion trap velocity map imaging spectrometer. The cryogenic ion trap produces a rotationally and vibrationally cold mass selected ion beam of Br2+, which simplifies the experimental spectra from vibrational hot bands and bands of mixed isotopic species. Vibrationally resolved photofragment excitation spectra are recorded for individual isotopologues of Br2+ (79Br2+, 79Br81Br+, 81Br2+) by [1+1] two-photon dissociation spectroscopy. Velocity map imaging of the photofragmented Br+ ions provides complementary information in the determination of spin-orbit states involved in corresponding electronic transitions. An experimental identification of the 14Σu- state has becomes possible based on the present experimental results and previously reported theoretical calculations. Vibrational analyses of the photofragment excitation spectra have yielded spectroscopic parameters, including state origins, harmonic frequencies, and anharmonic constants, for both A2Πu and 14Σu- states. The observed A2Πu state spin-orbit splitting and the "spin-forbidden" 14Σu--X2Πg transition band intensities indicate considerable spin-orbit couplings between the 14Σu- and A2Πu states. In addition, two groups of weak vibrational bands are also observed in the experimental spectra of 79Br81Br+, which may be due to symmetry-forbidden transitions from the X2Πg ground state to low-lying gerade states.

Competing pathways in the near-UV photochemistry of acetaldehyde.

Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265-328 nm. Ion images recorded probing CH3 radicals with single-photon VUV ionization show different dissociation dynamics in three distinct wavelength regions. At the longest photolysis wavelengths, λ > 318 nm, CH3 radicals are formed over tens of nanoseconds with a speed distribution that is consistent with statistical unimolecular dissociation on the S0 surface following internal conversion. In the range 292 nm ≤ λ ≤ 318 nm, dissociation occurs almost exclusively on the T1 surface following intersystem crossing and passage over a barrier, leading to the available energy being partitioned primarily into photofragment recoil. The CH3 speed distributions become bimodal at λ < 292 nm. In addition to the translationally fast T1 products, a new translationally slow, but non-statistical, component appears and grows in importance as the photolysis wavelength is decreased. Photofragment excitation (PHOFEX) spectra of CH3CHO obtained probing CH3 and HCO products are identical across the absorption band, indicating that three-body fragmentation is not responsible for the non-statistical slow component. Rather, translationally slow products are attributed to dissociation on S0, accessed via a conical intersection between the S1 and S0 surfaces at extended C-C distances. Time-resolved ion images of CH3 radicals measured using a picosecond laser operating at a photolysis wavelength of 266 nm show that product formation on T1 and S0via the conical intersection occurs with time constants of 240 ps and 560 ps, respectively.

Decomposing the First Absorption Band of OCS Using Photofragment Excitation Spectroscopy.

Photofragment excitation spectra of carbonyl sulfide (OCS) have been recorded from 212-260 nm by state-selectively probing either electronically excited S((1)D) or ground state S((3)P) photolysis products via 2 + 1 resonance-enhanced multiphoton ionization. Probing the major S((1)D) product results in a broad, unstructured action spectrum that reproduces the overall shape of the first absorption band. In contrast, spectra obtained probing S((3)P) products display prominent resonances superimposed on a broad continuum; the resonances correspond to the diffuse vibrational structure observed in the conventional absorption spectrum. The vibrational structure is assigned to four progressions, each dominated by the C-S stretch, ν1, following direct excitation to quasi-bound singlet and triplet states. The S((3)PJ) products are formed with a near-statistical population distribution over the J = 2, 1, and 0 spin-orbit levels across the wavelength range investigated. Although a minor contributor to the S atom yield near the peak of the absorption cross section, the relative yield of S((3)P) increases significantly at longer wavelengths. The experimental measurements validate recent theoretical work characterizing the electronic states responsible for the first absorption band by Schmidt and co-workers.

The ultraviolet photodissociation of CS2: The S(1D2) channel

The photodissociation of CS(2) has been investigated using velocity-map ion imaging of the S((1)D(2)) atomic photofragments following excitation at 193 nm and at longer wavelengths close to the S((1)D(2)) channel threshold. The experiments probe regions both above and below the energetic barrier to linearity on the (1)Σ(u) (+)((1)B(2)) potential energy surface. The imaging data in both regions indicate that the electronic angular momentum of the S((1)D(2)) atom products is unpolarized, but also reveal different dissociation dynamics in the two regions. Excitation above the barrier to linearity yields an inverted CS((1)Σ(+)) vibrational population distribution, whereas the long-wavelength state-to-state results following excitation below the barrier reveal CS((1)Σ(+))(v, J) coproduct state distributions which are consistent with a statistical partitioning of the energy. Below the barrier, photofragment excitation spectra point to an enhancement of the singlet channel for K = 1, relative to K = 0, where K is the projection of the angular momentum along the principal axis, in agreement with previous work. However, the CS cofragment product state distributions are found to be insensitive to K. It is proposed that dissociation below the barrier to linearity occurs primarily on a surface with a significant potential energy well and without an exit channel barrier, such as that for the ground electronic state. However, oscillatory structure is also observed in the kinetic energy release distributions, which is shown to be consistent with a mapping of parent molecule bending motion. This could indicate the operation of competing direct and indirect dissociation mechanisms below the barrier to linearity.

Mode specific photodissociation of CS2+via the A2Πu state: a time-sliced velocity map imaging study

The vibrationally mediated photodissociation of CS(2)(+) cations via the A(2)Π(u)(ν(1),ν(2),0) state has been studied by means of the velocity map ion imaging technique. The measurements were made with a double resonance strategy. The CS(2)(+) cations were prepared by a (3 + 1) resonance enhanced multiphoton ionization method. The photo-fragment excitation spectrum of S(+) was recorded by scanning the photolysis laser via the A(2)Π(u)(ν(1),ν(2),0) state. By fixing the photolysis laser wavelength at the specific vibrational state, the (1 + 1) photodissociation images of S(+) photofragments from numerous vibrationally mediated states have been accumulated. The translational energy release spectra derived from the resulting images imply that the co-fragments, CS radicals, are both vibrationally and rotationally excited. The one-photon photodissociation without the vibrational state selection has also been performed. Comparing the vibrationally mediated photodissociation with one-photon photodissociation observations, clear evidence of vibrational state control of the photodissociation process is observed.

Rotationally resolved spectroscopy of the à A21←X̃ B21 transition of H2S+ above the barrier to linearity using the mass-analyzed threshold ionization photofragment excitation technique

The A (2)A(1)<--X (2)B(1) transitions of H(2)S(+) above the barrier to linearity have been investigated with the energy resolution high enough to identify individual rotational transition lines for the first time. The rotational cooling of the cation is achieved either by the direct ionization or mass-analyzed threshold ionization (MATI) technique employed in the vacuum-ultraviolet laser excitation of the jet-cooled H(2)S. Subsequent photoexcitation leads to the H(2)S(+)-->H(2)+S(+) dissociation and the S(+) product yield taken as a function of the excitation energy gives the photofragment excitation (PHOFEX) spectra. The combined use of MATI and PHOFEX techniques greatly simplifies the spectrum allowing the accurate identification of the rotationally resolved bands which is otherwise a formidable task due to the intrinsic complexity of the A (2)A(1)<--X (2)B(1) transition. Highly excited states of A(0,7,0), A(0,8,0), and A(0,9,0) vibronic levels with different K quantum numbers which are located above the barrier to linearity are thoroughly investigated. The bent-to-quasilinear transition of H(2)S(+) above the barrier to linearity shows the characteristics of the Renner-Teller effect, showing the large A rotational constant and strong intensity borrowing of the highly vibrationally excited ground levels such as X(0,23,0) or X(0,24,0) in the dipole-allowed excitation. Spectroscopic parameters of term values, rotational, and spin-orbit coupling constants are precisely determined in this work, providing the most quantitative spectroscopic structure of the H(2)S(+) to date. Quantum-state dependent photodissociation dynamics are also discussed from spectral features of PHOFEX.

Intramolecular energy transfer and predissociation in C-state HCN

Using our ab initio configuration interaction potential energy surface for the C state of HCN, we carried out classical trajectory calculations aimed at simulating the predissociation dynamics of various vibronic states of HCN and DCN. Our results provide an interpretation of the experimental photofragment excitation spectrum of this species. They also indicate that tunneling, which had been postulated as a rate-determining step in the predissociation, is not substantial. Intramolecular energy transfer seems to be the slow step. Finally, there is evidence that on a time scale up to one rotational period, the predissociation kinetics is quite mode specific.

Determination of the O–IO bond dissociation energy by photofragment excitation spectroscopy

Pulsed laser photolysis combined with laser induced fluorescence of IO has been used to study the photofragment excitation spectrum of OIO. Analysis of the O–IO dissociation threshold observed around 480 nm provides the first experimental determinations of the OIO bond dissociation energy and heat of formation at 298 K, which are given by D 0 ∘ ( O – IO ) = ( 248.8 ± 1.0 ) kJ mol - 1 and Δ f H 298 ∘ ( OIO ) = ( 118.5 ± 2.0 ) kJ mol - 1 , respectively.

Attosecond nonlinear Fourier transformation spectroscopy of CO2 in extreme ultraviolet wavelength region

The interferometric autocorrelation functions of attosecond pulse trains in the time domain were measured by detecting CO(2) (2+) as well as the atomic and molecular fragment ions generated via two-photon absorption of intense vacuum ultraviolet-extreme ultraviolet light by CO(2). It was demonstrated that the Fourier transformation of the interferometric autocorrelation functions of the respective fragment ions appearing in a time-of-flight mass spectrum exhibit spectroscopic information in the frequency domain corresponding to the two-photon photofragment excitation spectra of CO(2) and the double ionization excitation spectrum to form CO(2) (2+).

Further investigation of the photodissociation dynamics of dichlorocarbene near 248nm

A further investigation of the 248 nm photodissociation of CCl(2), which expands upon our original study of this process [S. K. Shin and P. J. Dagdigian, Phys. Chem. Chem. Phys. 8, 3446 (2006)], is presented. The CCl(2) parent molecule and the CCl photofragment were detected by laser fluorescence excitation in a molecular beam experiment. From the dependence of the CCl(2) signals on the photolysis laser fluence, attenuation cross sections of the 0(0), 1(1), and 2(1) vibrational levels were determined; the cross sections for the excited vibrational levels were found to be significantly smaller than those for the ground vibrational level. The previously observed fragment CCl bimodal rotational state distribution was found to arise from the photolysis of more than one parent molecule. At low CHCl(3) mole fractions in the gas supplied to the pyrolysis beam source, it was concluded that CCl(2) is the photolysis precursor for both low-J and high-J CCl fragments. On the basis of the dependence of the CCl signals on the photolysis laser fluence, ground and vibrationally excited CCl(2), respectively, were assigned as the precursors to these two classes of fragments. The photofragment excitation spectra for low-J and high-J CCl fragments from the photolysis of CCl(2) were recorded in the wavelength range around 248 nm; both were found to be structureless. The 248 nm photodissociation dynamics of CCl(2) is discussed in light of our experimental observations and quantum chemical calculations of the CCl(2) excited electronic states.

Rovibronic spectrum of N2O+ ion at the A2Σ+ state

The rotational structures of the photofragment excitation spectrum of N2O+(A2Σ+) at high vibrational levels have been studied experimentally. The parent N2O+(X2Π) ions were prepared by (3+1) resonance-enhanced multiphoton ionization of jet-cooled N2O molecules by 36055nm laser beam，and were excited by another laser to the predissociative A2Σ+ state in the range of 280—320nm. Two types of rotational transition，i.e.，2Σ+←2Π and 2Π←2Π，have been clearly observed for a series of vibronic transitions. The rotational constants and spin splitting constants have been obtained from the spectral analysis.

K-Dependent Predissociation Dynamics of CS2 in the 210−216 nm Region

The dependence of CS2 predissociation upon rotational quantum number K at vibrational levels below the barrier to linearity of the 1B2(1Sigmau+) state has been investigated in detail with laser spectroscopy, by using a heated supersonic source to increase the intensities of hot band transitions. Predissociation lifetimes were determined from rotational contour simulations of 13 vibronic bands in the CS photofragment excitation (PHOFEX) spectrum, each terminating at the same upper vibrational level but via transitions with different K number (K = 0, 1, 2, respectively). The rovibrational populations of CS fragment at these excitation bands were derived from the laser-induced fluorescence (LIF) spectrum, and were used further to obtain the dissociation branching ratios S(1D)/S(3P) as well as the excess energy partitionings after dissociation. The lifetimes and the branching ratios were found to be sensitively dependent on quantum number K; the lifetime decreases with the increase of K, and the branching ratio increases with K. Analysis shows that quantum number K influences the S(1D) channel more effectively than the S(3P) channel. About 28 and 15% of the total available energy is taken up by the CS vibrational and rotational degrees of freedom, respectively. Systematic analysis indicates that the two electronic states interacting with 1B2(1Sigmau+) state should be bent, and the state correlating with S(1D) channel should be more bent.

Velocity Imaging Studies on Ion-Pair Dissociation of CH3Br + hνVUV → CH3+ + Br- as a Function of Wavelength

The ion-pair dissociation dynamics of CH3Br + hνVUV → CH3+ + Br- as a function of vacuum ultraviolet (VUV) wavelength have been studied by means of velocity imaging. Photofragment excitation (PHOFEX) spectra as well as images for both the cation and the anion products have been collected, which confirm the dissociation occurs through the ion-pair channel from the momentum matching of both products. The wavelength region for the PHOFEX covers a small portion (from 121.4 to 122.7 nm) of the absorption spectrum where transitions from the ground state to both ns and np Rydberg states as well as to the underlying continuum states are all present. Images were taken at selected wavelengths both on the peaks and in the valleys of the PHOFEX spectrum, as well as at 118.22 nm. The PHOFEX spectra closely resemble the absorption spectra of CH3Br, and the images taken at peaks and continuum wavelengths in PHOFEX show no marked differences with respect to the product energy and angular distributions. These observations prompt us to conclude that the ion-pair state is directly populated by the VUV photons and the ion-pair dissociation is one of the major exit channels for the excited states in the energy region just below the ionization energy of the parent molecule, although large uncertainties in wavelength in the absorption spectra make the proposed mechanisms less conclusive.

Photodecomposition reaction of trapped aniline cation by photofragment excitation spectroscopy

A photofragment excitation spectrum of aniline cation, C 6 H 5 NH 2 + , was measured in the range of 450–400 nm by ion-trap mass spectrometry. The trapped cations exposed to a nanosecond laser field ( I ≈ 1 MW/cm 2, Δ t ≈ 10 ns) absorbed two photons and dissociated into C 5 H 6 + efficiently in the 427–417 nm range. From a comparison with the photoelectron spectrum of neutral aniline, it is identified that the photodissociation of aniline cation into C 5 H 6 + proceeds via the second electronically excited ( B ˜ 2 B 1 ) state.

The spectral assignment for the A2Σ+←X2Π transition of N2O+ ions

The spectra of N2O+ ions in A2Σ+ state have been investigated. Pure parent N2O+ ions, in the X2Π3/2,1/2(000) state, were prepared by (3+1) multiphoton ionization of jet-cooled N2O molecules using a laser beam at 360.55 nm. By introducing another laser, the parent ions were excited to the predissociative A2Σ+ state, and the fragment NO+ was detected by a time-of-flight mass spectrometer. The NO+ photofragment excitation (PHOFEX) spectra were recorded by scanning the dissociation laser in the wavelength range of 278—328 nm. The PHOFEX spectra could be attributed completely to the A2Σ+←X2Π transition of N2O+, in which most vibronic bands were observed for the first time. By considering the Fermi resonance between the ν1 and ν2 modes, the spectra were assigned, and the spectral constants, such as vibrational frequencies, anharmonic constants, and Fermi interaction constant, were obtained with relatively high reliability and precision.

Spectroscopic study of N2O+(A 2Σ+) by photofragment excitation spectrum

Photofragment NO+ excitation spectrum of N2O+ ions has been studied in the wavelength range of 278–328 nm, where the parent N2O+ ions were state-selectively prepared at the X 2Π1/2,3/2(000) levels by [3+1] multi-photon ionization of jet-cooled N2O molecules at 360.55 nm. The spectrum was attributed completely to the A 2Σ+←X 2Π3/2,1/2(000) electronic transition of N2O+. Totally 47 vibronic bands associated to 24 vibrational levels of the A 2Σ+ state were identified in the present work, most of which were observed for the first time. The sufficient spectroscopic data made it possible to investigate the Fermi-resonance between the ν1 and ν2 vibrational modes at the A 2Σ+ state. Based on the assignment, the spectral constants of the A 2Σ+ state, such as vibrational frequencies, anharmonic constants, and Fermi interaction constant, were determined with relatively high reliability and precision.

Laser-initiated processes within (SO2)m(NO)n weakly-bound clusters

The SO2 molecule embedded in (SO2)m(NO)n weakly-bound clusters, may be pumped to the triplet state, without ionizing the clusters, using the third harmonic of a Nd:YAG laser. The fast electronic energy dissipation increases the internal energy of the clusters, thereby causing their fragmentation. The analysis of the NO fragment spectra reveals that the electronic energy is randomized within the clusters prior to their fragmentation and that the “temperature” increases from 10 to 250 ± 10 K. During the irradiation of the clusters with tunable laser light in the 222 to 235 nm it is observed the formation of NO+, S+, SO+, (SO2)SO+ and SONO+. The photofragment excitation (PHOFEX) spectra of S+, SO+, (SO2)SO+ display the SO2 C̃1B2←X̃1A1 structure, whereas the PHOFEX spectra of SONO+ presents the NO A2Σ←X2ΠΩ structure. On the other hand, the non-resonant irradiation of the clusters at 266 nm, besides NO+, S+, and SO+, produces SO2+ and SO2NO+.

Photodissociation of methyl iodide clusters in the A-band excitation: Photofragmentation excitation spectra of (CH3I)n by ultraviolet pump-CRD probe measurement

Methyl iodide clusters (CH3I)n in a supersonic jet are photolyzed in an excitation of the valence A band with a tunable ultraviolet light source, and the iodine molecule (I2) thus produced is detected by cavity ring-down (CRD) spectroscopy. A production yield of I2 is measured in the region of 245–275 nm to obtain photofragment excitation (PHOFEX) spectra of (CH3I)n. The PHOFEX spectra show maximum at 250 nm (∼1000 cm−1 blueshifted from monomer) at low concentration, and a new peak at 260–265 nm (∼450 cm−1 redshifted) at high concentration. These two peaks are assigned to dimer and higher cluster (hexamer to decamer), respectively, based on our infrared-CRD study [Chem. Phys. 286, 337 (2003)]. The redshifted peak of the higher clusters indicates that perturbation of the excited state due to intermolecular interaction is stronger than in the dimer.