Infrared Laser Assisted Photofragment Spectroscopy Research Articles

Conformational dependence of intramolecular vibrational redistribution in methanol

Previous state-selected spectra of methanol in the 5nu(1) OH stretch overtone region [O. V. Boyarkin, T. R. Rizzo, and D. S. Perry, J. Chem. Phys. 110, 11346 (1999)] revealed a structure indicating an intramolecular vibrational redistribution on three time scales. Whereas in that work, methanol in the 5nu(1) bright state was prepared close to the staggered conformation, methanol in the "partially eclipsed" conformation is prepared here by double resonance excitation through a torsionally excited intermediate state. The excited molecules are detected by infrared laser assisted photofragment spectroscopy. In partially eclipsed methanol, the strong coupling of the nu(1) OH stretch to the nu(2) CH stretch becomes weaker, but the coupling responsible for the widths of the narrowest features becomes stronger.

Nonlinear intensity dependence in the infrared multiphoton excitation and dissociation of methanol pre-excited to different energies

We report quantitative dissociation yields for the reaction CH3OH (vOH)→ lim nhνCH3+OH induced by infrared multiphoton excitation of methanol pre-excited to various levels of the OH stretching vibration (vOH=0, 1, 3, 5). The yields are measured by detecting OH using laser induced fluorescence. It is demonstrated that for low levels of pre-excitation (vOH=0, 1, 3) there is a substantial nonlinear intensity dependence, as a higher yield is found for self mode-locked CO2 laser pulses (with higher peak intensity) as compared to single mode pulses of the same laser fluence, but lower peak intensity. In contrast, at high levels of preexcitation (vOH=5) this nonlinear intensity dependence is absent. Quantitative model calculations are carried out using a case B/case C master equation approach that takes nonlinear intensity dependence into account. The calculations are consistent with the experimental results and confirm the prediction that an important part of the selectivity of the CO2 laser excitation step in infrared laser assisted photofragment spectroscopy of CH3OH is due to this nonlinear intensity dependence. We discuss further consequences of these experimental observations and theoretical predictions, which are also extended to infrared multiphoton excitation of C2H5OH. Infrared (C–O) chromophore band strengths are reported for CH3OH and C2H5OH.

Rotational and Torsional Analysis of the OH-Stretch Third Overtone in 13CH3OH

We record double resonance spectra of the 4ν1 band of jet-cooled 13C-methanol using single rotational state selection in the ν1 fundamental and subsequent promotion of the selected molecules to the fourth vibrational level. We then detect transitions to the final excited states by infrared laser assisted photofragment spectroscopy (IRLAPS). The assigned A symmetry transitions reach upper states with K=0 and 1, and J from 0 to 5. For E symmetry, the transitions reach levels with K in the range −3 to 2 and J from 1 to 7. The rotation–torsional analysis determines a value for the torsional tunneling splitting of 2.8±0.4 cm−1 at v1=4. In a previous paper (J. Chem. Phys.110, 11 359–11 367 (1999)), we reported a trend of monotonically decreasing tunneling splittings in 12CH3OH for v1=0, 3, and 6 that we explained by a model that incorporates a linear increase in the torsional barrier height with OH stretch excitation. The 13CH3OH tunneling splitting for the 4ν1 band is in quantitative agreement with the trend found for 12CH3OH.

Torsion–rotation analysis of OH stretch overtone–torsion combination bands in methanol

We report rotationally resolved spectra of jet-cooled methanol for the OH stretch overtones, 2v1 and 3v1, and for the torsional combinations, 2v1+v12, 2v1+2v12, 3v1+v12, and 3v1+2v12. The spectra are obtained by direct excitation from the vibrational ground state with an infrared laser pulse. Population in the resulting upper state levels is detected by infrared laser assisted photofragment spectroscopy (IRLAPS). Global fits of the spectra to the Herbst Hamiltonian yield the torsional and rotational parameters, including F, ρ, V3, and V6, for each OH stretch excited state. For each quantum of OH stretch excitation, we find that the torsional barrier height V3 increases by 40.9±1.9 cm−1 and the torsional inertial F decreases by 0.89±0.02 cm−1. With reference to ab initio calculations, we explain the increase in V3 in terms of changes in the electronic structure of methanol as the OH bond is elongated. For Δv12=1 we observe only transitions with ΔK=±1, and for Δv12=2 we observe only ΔK=0. We present a Franck–Condon model to explain these apparent selection rules and the overall pattern of intensity.

Vibrational dependence of the torsional tunneling splitting and the a to b intensity evolution in the OH overtone spectra of CH3OH

An ab initio investigation of the OH-stretching overtone of methanol is presented aimed at interpretation of two of the interesting features reported in recent supersonic-jet infrared-laser-assisted photofragment spectroscopy (IRLAPS) studies, namely, (i) a monotonic decrease of the torsional E – A splitting as υOH increases, (ii) an evolution of the a-type transition moment towards dominance at higher excitations of the OH-stretching vibration. At the MP2 level with a 6-311+G(3df,2p) basis set, one-dimensional functions for the potential energy, dipole moment (a and b directions), barrier height V3, and torsional constant F have been mapped out as a function of the OH-bond length with the remaining structure relaxed. All ab initio quantities have been expressed as Taylor-series expansions in the dimensionless OH normal coordinate, q. Calculations have been carried out in the harmonic basis set to yield vibrational energies and eigenfunctions. The latter have been used to evaluate the torsional parameters (V3, F) and the resulting E – A torsional splittings at each OH excitation, and to compute the evolution of the infrared transition intensity ratio [Formula: see text]a/b. The ab initio results reproduce the reported decrease in E – A splitting by a factor of 5 in going from υOH = 0 to υOH = 6, with a maximum deviation of 19% at the highest excitation. The calculated variation in [Formula: see text]a/b is in qualitative agreement with the experimental trend, showing the observed sharp rise at high υOH. PACS Nos.: 33.20E, 33.80B

Vibrational dependence of the torsional tunneling splitting and the <i>a</i> to <i>b</i> intensity evolution in the OH overtone spectra of CH<sub>3</sub>OH

An ab initio investigation of the OH-stretching overtone of methanol is presented aimed at interpretation of two of the interesting features reported in recent supersonic-jet infrared-laser-assisted photofragment spectroscopy (IRLAPS) studies, namely, (i) a monotonic decrease of the torsional E – A splitting as υOH increases, (ii) an evolution of the a-type transition moment towards dominance at higher excitations of the OH-stretching vibration. At the MP2 level with a 6-311+G(3df,2p) basis set, one-dimensional functions for the potential energy, dipole moment (a and b directions), barrier height V3, and torsional constant F have been mapped out as a function of the OH-bond length with the remaining structure relaxed. All ab initio quantities have been expressed as Taylor-series expansions in the dimensionless OH normal coordinate, q. Calculations have been carried out in the harmonic basis set to yield vibrational energies and eigenfunctions. The latter have been used to evaluate the torsional parameters (V3, F) and the resulting E – A torsional splittings at each OH excitation, and to compute the evolution of the infrared transition intensity ratio a/b. The ab initio results reproduce the reported decrease in E – A splitting by a factor of 5 in going from υOH = 0 to υOH = 6, with a maximum deviation of 19% at the highest excitation. The calculated variation in a/b is in qualitative agreement with the experimental trend, showing the observed sharp rise at high υOH. PACS Nos.: 33.20E, 33.80B

Intramolecular energy transfer in highly vibrationally excited methanol. II. Multiple time scales of energy redistribution

State-selected spectra of the OH stretch overtones of methanol in the range of v1=3–8 reveal spectral splittings and broadenings that result from vibrational couplings within the molecule. We employ a two-color excitation technique in which an infrared pulse promotes jet-cooled methanol molecules to a single rotational state in v1=1 or 2 and a second visible or near-infrared laser pulse is scanned to record a vibrational overtone spectrum. The final vibrationally excited species are detected by infrared laser assisted photofragment spectroscopy. The implications of the spectra for vibrational dynamics in the time domain can be understood in terms of a hypothetical coherent excitation of relevant portions of the spectrum. The observed splittings and widths correspond to three time scales. The largest splittings imply subpicosecond oscillation of energy between the OH stretch and a combination with the C–H stretch (5ν1⇔4ν1+ν2 and 6ν1⇔5ν1+ν2) or a combination with the COH bend (7ν1⇔6ν1+2ν6). Secondary time scales correspond to finer splittings and are thought to arise from low-order resonances with other vibrational states. We argue that the nonmonotonic energy dependence of the presence and extent of such secondary structure throughout the recorded spectra reflects the requirement of resonance with important zeroth-order states. The third time scale, represented by the widths of the narrowest features at each overtone level, reflects the onset of vibrational energy randomization. These widths increase exponentially with vibrational energy in the range 2ν1 up to 8ν1. At the highest energy (25 000 cm−1) the three time scales begin to converge, implying an irreversible decay of the OH stretch overtone in 300 fs.

Intramolecular energy transfer in highly vibrationally excited methanol. III. Rotational and torsional analysis

We report here torsional analysis of rotationally resolved spectra of the 3ν1, 5ν1, and 6ν1 (OH stretch) bands of jet-cooled methanol. The upper states are reached by a double resonance excitation scheme involving the selection of single rotational states in the ν1 fundamental band. Detection of the overtone transitions (nν1←ν1) is by infrared laser assisted photofragment spectroscopy (IRLAPS). The torsional tunneling frequency declines monotonically from 9.1 cm−1 in the vibrational ground state to 1.6 cm−1 at 6ν1. For the available rotational levels at 3ν1 (K=0–3) and 6ν1 (K=0,1), the pattern of torsional energies is approximately regular. To obtain the vibrational dependence of the torsional barrier V3, it was necessary to use the OH radical and HOOH as models for the vibrational dependence of the torsional inertial constant F. The assumed linear dependence of V3 on ν1 accounts for the torsional tunneling splittings at v1=0, 3, and 6 and for the pattern of the torsional energies. V3 increases by 40–45 cm−1 per quantum of OH excitation.

Intramolecular energy transfer in highly vibrationally excited methanol. I. Ultrafast dynamics

Vibrational overtone excitation of jet-cooled methanol, in combination with infrared laser assisted photofragment spectroscopy (IRLAPS) detection, reveals OH stretch bands that are significantly simplified with respect to room-temperature spectra. The simplification afforded by jet-cooling permits the observation of spectral splitting on the order of 50 cm−1 in the region of the 5ν1 OH stretch overtone band. Tracking this splitting as a function of OH stretch vibrational level in combination with isotopic substitution studies allows us to identify the perturbing state as the combination level involving four quanta of OH stretch and one quantum of CH asymmetric stretch, 4ν1+ν2. Careful examination of the spectra reveals that this strong interaction arises from a fourth-order anharmonic term in the Hamiltonian that couples the OH and CH ends of the molecule. These frequency domain results indicate that subsequent to coherent excitation of the 5ν1 band, methanol would undergo energy redistribution to the methyl part of the molecule on a time scale of ∼130 fs. This work also suggests that similar strong resonances may occur more generally in molecules that possess two different high-frequency oscillators in close proximity.

Secondary time scales of intramolecular vibrational energy redistribution in CF3H studied by vibrational overtone spectroscopy

Rotationally resolved vibrational overtone spectra of several CH stretch bands of CF3H have been obtained using a combination of jet-cooling, infrared-optical double-resonance, and infrared laser assisted photofragment spectroscopy detection. The high selectivity and sensitivity of this combination allows measurement of spectral splittings that determine the secondary time scales of vibrational energy redistribution. The spectroscopic results show that in the energy region of the vCH=3–5 CH stretch levels, the vibrational energy would remain localized in the mixed CH stretch–bend states on time scales longer than 13–130 ps if coherently excited. This is as much as 2500 times longer than the time scale for energy transfer between the CH stretch and the HCF bend vibrations. Moreover, the results confirm that the energy redistribution dynamics are sensitive to the character of the bath states and not simply to the total vibrational energy.

Rotational state selected vibrational overtone spectroscopy of jet-cooled molecules

The combination of infrared-optical double resonance excitation with infrared laser assisted photofragment spectroscopy (IRLAPS) detection permits the measurement of rotationally resolved vibrational overtone spectra of jet-cooled CF3H. The IRLAPS detection technique is sufficiently selective to be able to discriminate against the dissociation of molecules in the ground state and the vCH=1 level used as an intermediate in the double resonance scheme. The combination of this selective detection technique with both supersonic cooling and double resonance produces J state resolved overtone spectra of the 42 vibrational overtone band. These spectra put a limit on the homogeneous linewidths of the rovibrational transitions of ∼0.5 cm−1.

G. Intramolecular Vibrational‐Rotational Redistribution, Anharmonic and Rovibronic Interactions. Vibrational Overtone Spectra of Jet‐Cooled CF3H by Infrared Laser Assisted Photofragment Spectroscopy

AbstractUsing our recently developed technique of infrared laser assisted photofragment spectroscopy (IRLAPS), we have measured four vibrational overtone bands of jet‐cooled CF3H in the N = 4 and N = 5 CH stretch‐bend polyads. The cooling in the expansion eliminates much of the rotational congestion and reveals homogeneous structure which reflects the vibrational coupling of the stretch bend states with other modes of the molecule. This homogeneous structure indicates that the coupling of the stretch‐bend states to the rest of the molecule is 20–2000 times weaker than the stretch bend coupling itself and implies a hierarchy of time scales in the vibrational energy redistribution of the excited molecule. The overtone spectra of the jet‐cooled molecules also reveal a wide variation in the coupling strengths of the stretch‐bend states to the other modes, suggesting that the coupling depends upon the presence of specific low‐order resonances rather than on the total density of states.