Itamar Burak

State‐resolved photodissociation of OCS monomers and clusters

Photodissociation of OCS in the region from 222–248 nm has been investigated by monitoring the CO and S(1D2) primary photoproducts; as well as the secondary production of S(3P2), S(3P1), and S(3P0) using fluorescence induced by a tunable vacuum ultraviolet laser source based on four‐wave mixing in magnesium vapor. The quantum yield of S(3P) was found to be 0.00±0.02 at 222 nm. Thus, in contrast to our preliminary report, the present more detailed investigation shows that the sole sulfur product appears to be S(1D). The CO photofragment is produced almost exclusively in v=0 [CO(v=1)/ CO(v=0)≤0.02], but the rotational distribution is inverted and peaked at very high rotational levels. The peak shifts from J=56 for dissociation at 222 nm to J=31 at 248 nm. Doppler profiles of the CO rotational transitions reveal (1) that all observed levels are produced in coincidence with S(1D), (2) that for 222 nm photolysis the fragment recoil anisotropy shifts from a distribution characterized by β=1.9 at J=67 toward one...

State‐to‐state photodissociation dynamics of trans‐glyoxal

The photodissociation of glyoxal has been investigated by monitoring the CO internal energy distribution using tunable vacuum ultraviolet laser‐induced fluorescence on the A←X system. Appearance times for the CO are in excellent agreement with the glyoxal fluorescence decay times, indicating that there is no long‐lived intermediate in the dissociation. The quantum yield for CO production is independent of the K quantum number describing the glyoxal rotation. The CO is formed almost entirely in v=0 but is spread over a broadly excited rotational distribution peaking at J≂42. Analysis of the CO Doppler profiles shows that the velocity of the CO increases with increasing rotational level and that the CO recoil velocity vector is oriented predominantly perpendicular to its angular momentum vector. These observations, which are in agreement with both previous time‐of‐flight data and molecular orbital calculations, are consistent with a model for the dissociation involving planar intermediates for the two chann...

Angular correlations between recoil velocity and angular momentum vectors in molecular photodissociation

A technique has been developed for determining the angular correlation between a photofragment’s angular momentum vector J, its recoil velocity vector v, and the transition dipole moment of the parent molecule μp . Doppler profile spectroscopy used in conjunction with laser‐induced fluorescence probing by polarized light can be used to determine the correlations. The pairwise correlations between these vectors as well as their triple correlation are discussed for limiting cases using a classical approach as well as for the general case using a quantum approach based on density matrices. The current formulations differ in two ways from the recent approach of Dixon, who used a bipolar expansion of the correlated velocity and angular momentum distributions. The physical basis for the influence of the vector correlations on the Doppler profile is somewhat more transparent in the current formulations, and the direct connection between the measured correlations and the t‐matrix elements occurring in the theory ...

Mechanism of passive Q switching in CO 2 lasers

Passive Q switching of a CO 2 laser by saturable absorbers is analyzed in terms of a four-state kinetic model previously used to interpret infrared saturations double resonance, and pulse transmission. Good agreement is found between the predictions of this model and the experimental dependence of such variables as pulsewidth, repetition frequency, and peak power on the presence of buffer gases mixed in with the absorber.

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....

Vibration-vibration energy transfer in the v 3 mode of CO 2

The vibration-vibration energy transfer of CO 2 gas initially excited to the first asymmetric stretch level (0001) has been observed. Collisional pumping to the (0111) combination level is measured by monitoring the fluorescence due to the (0111) → (0110) band. The rate constant for the process: CO 2 (0111) + CO 2 (0000) → CO 2 (0001) + CO 2 (0110) is found to be (5.3 \pm 1) \times 10^{6} s-1torr-1.

Tea chemical lasers from H2 + Cl2 and H2 + Br2

Abstract HCl and HBr chemical lasers based on the reactions H + Cl 2 and H + Br 2 have been successfully operated using a transverse discharge excitation. Both lasers yielded outputs in the kW range and pulse widths of 200–1000 nsec. Oscillations were obtained on a large number of rotation-vibration lines in the 3-2, 2-1 and 1-0 vibrational bands. Self mode locking of the laser has been observed on most transitions. Oscillations on 1-0 transitions are observed at H 2 :Cl 2 and H 2 :Br 2 ratios greater than 10:1. The main characteristics of the lasers are discussed in terms of the reaction and initiation mechanism.

CO product distribution from metastable levels of HCO X̃ 2A’ prepared by stimulated emission pumping

Vibrational and rotational distributions have been measured for the CO produced following dissociation of HCO prepared in metastable ground‐state levels by stimulated emission pumping. The metastable levels are characterized by high vibrational excitation in the HC–O stretching coordinate. The CO (v=2) rotational distribution following dumping of population on the QR0(5) transition to the (0, 7, 0) level is rotationally cold and its distribution is in very good agreement with the theoretical prediction by Dixon. The linewidths for stimulated emission pumping transitions to the (0, 7, 0) and (0, 8, 0) levels are 1.0 and 1.6 cm−1, respectively. Since it is possible to prepare a wide variety of metastable HCO levels whose wave functions sample different regions of the HCO potential energy surface, measurement of the dissociation dynamics of HCO so prepared should provide a detailed picture of the potential surface at energies from 0–10 000 cm−1 above the dissociation limit.

The 157 nm photodissociation of OCS

The photodissociation of OCS at 157 nm has been investigated by using tunable vacuum ultraviolet radiation to probe the CO and S photoproducts. Sulfur is produced almost entirely in the 1S state, while CO is produced in its ground electronic state and in vibrational levels from v=0–3 in the approximate ratio (v=0):(v=1):(v=2):(v=3)=(1.0):(1.0):(0.5) :(0.3). The rotational distribution for each vibrational level is found to be near Boltzmann, with temperatures that decrease from 1350 K for v=0 to 780 K for v=3. Measurements of the CO Doppler profiles demonstrate that the dissociation takes place from a transition of predominantly parallel character (β=1.8±0.2) and that the CO velocity and angular momentum vectors are perpendicular to one another.

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.