Scott H. Kable

Photodissociation dynamics of acetone at 193 nm: Photofragment internal and translational energy distributions

The photofragment internal and translational energy distributions resulting from the 193 nm photolysis of acetone have been measured. Vacuum‐ultraviolet laser‐induced fluorescence was used to probe the CO fragment, and multiphoton ionization time‐of‐flight mass spectrometry was used to probe the CH3. A Boltzmann distribution was observed to fit each degree of freedom with the following characteristic temperatures: CO: Tvib =2700 K, Trot =3000 K, Ttrans =3000 K; CH3: Tvib =800 K, Trot =500 K, Ttrans =3500 K. No evidence was found for two distinct CH3 populations, as might be characteristic of a stepwise reaction. Energy partitioning between the fragments was fit well by a simple impulsive model in which the available energy is divided equally between the two dissociating C–C bonds, the two bonds cleaving in rapid succession on a time scale short enough to allow little redistribution of energy into the methyl degrees of freedom.

The S1–S0(1B2u–1Ag) transition of p‐difluorobenzene cooled in a supersonic free jet expansion. Excitation and dispersed fluorescence spectra, vibrational assignments, Fermi resonances, and forbidden transitions

The vibronic spectroscopy of the S1(1B2u)–S0(1Ag) transition of p‐difluorobenzene (000 at 36u2009838 cm−1) cooled in a supersonic free jet expansion in argon has been reinvestigated in some detail. Analysis of over 50 vibronic transitions using fluorescence excitation and dispersed single vibronic level fluorescence spectroscopy has led to the establishment or confirmation of the assignments of 19 S1 and S0 frequencies, including eight previously unassigned S1 vibrational frequencies, and the reassignment of two S1 and one S0 frequencies. Several Franck–Condon forbidden transitions have been identified. Their activity in the S1–S0 spectrum is attributed to vibronic coupling involving higher lying electronic states. Forbidden transitions involving b3g modes, notably ν27 and ν26, derive their intensity from a higher lying 1B1u electronic state, via vibronic coupling that is analogous to that responsible for the 1B2u–1Ag transition in benzene. Numerous Fermi resonances in both the S1 and S0 states have been iden...

Photofragment excitation spectroscopy of the formyl (HCO/DCO) radical: Linewidths and predissociation rates of the Ã(A‘) state

Photofragment excitation (PHOFEX) spectra of the jet‐cooled formyl (HCO and DCO) radical have been measured by monitoring laser‐induced fluorescence of the CO fragment. The following A(A‘)←X(A’) vibronic transitions were measured: (v1,v2,v3)←(0,0,0) where for HCO v2=6–16 with v1,v3=0; v2=7–12 with v1=1, v3=0; v2=9–12 with v1=0, v3=1 and for DCO v2=14–18 with v1,v3=0. The PHOFEX technique provides a powerful method for discriminating against strong transitions so as to allow assignment and measurement of many weaker Franck–Condon bands. The linewidths of the diffuse transitions lie in the 16–150 cm−1 (FWHM) range; they increase strongly with K’ (the projection of the total angular momentum onto the a axis) and decrease slightly with v’2 (the number of HCO/DCO bending quanta). The linewidths are interpreted as radiationless transition rates and yield upper A‐state lifetimes ranging from 70 to 700 fs. These data are in excellent agreement with a dynamical model of HCO/DCO curve crossing developed by R. N....

Dissociation dynamics of C3O2 excited at 157.6 nm

The dissociation of carbon suboxide by single photon absorption at 157.6 nm has been studied under the collisionless environment of a molecular beam. The primary products are 2CO+C [3P(97%) or 1D(3%) ]. The spin–orbit levels of the 3P carbon are statistically distributed. The CO rotational populations in the first four vibrational levels are found to be well described by Boltzmann distributions with temperatures 3430, 4120, 4670, and 2340 K for υ=0,1,2,3, respectively. A second low temperature component in the υ=0 rotational distribution is attributed to CO produced in coincidence with C(1D). Significant population is found in the first four vibrational levels with less than 3% estimated in the higher levels; a vibrational temperature of 3700 K fits the distribution. Analysis of the Doppler profiles of the CO and carbon suggest that the dissociation is stepwise; the first dissociation appears to be described by an anisotropy parameter near β=2, while the second appears to be isotropic. The mean CO fragmen...

CO product distributions from the visible photodissociation of HCO

The final state distribution of carbon monoxide produced in the photodissociation of the formyl (HCO) radical has been studied both experimentally and theoretically. Renner–Teller coupling between the excited HCO A state and the ground state leads to dissociation and yields H and CO. Vibrational and rotational distributions have been measured for CO produced after excitation to specific vibrational levels on the A←X transition of HCO cooled in a supersonic expansion. The strongest transitions are for excitation to vibrational states with six to 16 quanta in the bending mode, and dissociation from these states produces inverted CO rotational distributions with average rotational quantum numbers <J≳ in the 22–33 range. The value of <J≳ increases monotonically with the vibrational quantum number describing the bend of the excited triatomic. Experiments involving excitation of one quantum of the C–H stretching motion have revealed that this vibration results in increased rotational excitation of the product CO with values of <J≳ as high as 41. In contrast, experiments indicate that the C–O stretching mode of HCO acts nearly as a spectator during the dissociation process. Excitation of HCO states with one quantum of C–O stretch yields vibrationally excited CO as the dominant dissociation product, but with a rotational distribution similar to that for CO(ν=0) produced following the excitation of HCO states without the quantum of C–O stretch. Classical trajectory calculations on an ab initiopotential energy surface have modeled many of the experimental features and trends of the CO product distributions. There are, however, some discrepancies in the positions of rotational maxima and in the efficiency of the coupling of the C–O vibration of HCO to the dissociation coordinate. It is not clear whether these are due to approximations made in the modeling or inaccuracies in the potential energy surface.

Collision partner and level dependence of vibrational relaxation in S0p‐difluorobenzene. Stimulated emission pumping combined with single vibronic level fluorescence spectroscopy

Rate coefficients have been measured for vibrational relaxation in S0 p‐difluorobenzene vapor under assault from eight collision partners. The dissipation of vibrational energy is monitored from each of four vibrational states with energies in the range ∼1500–3300 cm−1. Rate coefficients are found to vary between 1.5×10−10 cm3u2009molecule−1u2009s−1 for helium colliding with p‐difluorobenzene in the lowest state explored, to 12×10−10 cm3u2009molecule−1u2009s−1 for p‐difluorobenzene self‐relaxation from the highest level studied. The dependence of collision partner on the vibrational relaxation efficiency for each initially prepared state is studied, revealing a dependence on the reduced mass of the collision pair, and/or the well depth of the system. In addition the dependence of the efficiency on the initially prepared state is investigated for any one collision partner using a modified version of the Parmenter–Tang propensity rule model. A comparison between these data for vibrational relaxation in the S0 state of p‐di...

The photochemistry of the formyl radical: Energy content of the photoproducts

Premiere determination de la distribution denergie dans les photofragments dun radical intermediaire. Mesure des largeurs des bandes de HCO diffuses correspondant aux niveaux K = 1,2,3

Product distributions in the 157 nm photodissociation of CO2

The vibrational and rotational distributions of CO(1Σ+g) produced in the 157 nm photodissociation of CO2 have been determined by measuring vacuum‐ultraviolet laser‐induced fluorescence spectra of the CO photoproduct. The photodissociation of CO2 is known to occur via two pathways; one yielding O(1D) and the other yielding O(3P). Spin conservation and previous experimental studies confirm that dissociation via the O(1D) channel is the dominant process. The available energy for this channel is sufficient to populate only the ground and first excited vibrational levels of CO. We measured the rotational distributions for CO in v=0 and v=1 and found them to be nonBoltzmann. In fact, a highly structured distribution with distinct peaks at J=10, 24, 32, and 39 is observed for CO in v=0. A less structured population is displayed by molecules in v=1. The relative vibrational population (v=0/v=1) was determined to be 3.7±1.2. Doppler spectra of individual rovibronic transitions were also recorded. The profiles have...

COLLISIONAL EXCITATION OF CO BY 2.3 EV H ATOMS

Vibrational and rotational distributions of CO excited by collisions with 2.3 eV H atoms have been obtained by monitoring the products with vacuum ultraviolet (VUV) laser induced fluorescence. Translational‐to‐vibrational (T→V) transfer is dominated by the dynamics of collisions occurring in the two wells on the H+CO potential energy surface, one characterizing the HCO radical and the other characterizing COH. The measured vibrational distributions agree well with the results of trajectory calculations performed on the ab initio potential energy surface of Bowman, Bittman, and Harding (BBH). The measured rotational distributions show two significant differences from the calculated ones. First, for v=0 the experiments find more population in J<15 than predicted. This discrepancy may be due to errors in the repulsive part of the BBH surface that is outside the HCO and COH wells, but inside the van der Waals well. Second, for v=1, the experimental distribution is flat from J=0 to J=10, whereas the calculated...

Translational temperature dependence of mode‐to‐mode vibrational energy flow in 1B3u naphthalene induced by low energy collisions with Ar

Mode‐to‐mode vibrational energy flow from the 81 (b1g) level in the 1B3u state of naphthalene (C10H8) has been mapped in the energy regime evib≤800 cm−1. Vibrational state changes are induced by low energy collisions with the carrier gas Ar in the warm to cool regions of a supersonic expansion. The pattern of energy transfer is estimated from time‐resolved dispersed fluorescence spectra obtained following laser excitation of the absorption transition 810. Propensities for particular transfer channels are found to be in qualitative accord with expectations based on studies of single‐ring aromatics such as benzene and aniline. One‐quantum changes are preferred over two‐quantum changes and an energy gap law is evident. The competition between certain vibrational energy transfer channels is examined as a function of the translational temperature Ttrans at specific distances X/D from the nozzle aperture. At X/D≤1.5, evidence is found for both endoergic and exoergic transfer channels. Ttrans at X/D=1.5 is det...