Nathan Presser

Detection of Cl atoms and HCl molecules by resonantly enhanced multiphoton ionization

Abstract Resonantly enhanced multiphoton ionization has been used to detect Cl(3p 2P 3 2 o) and Cl*(3p 2P 1 2 o) via the pumping of two-photon-allowed transitions in the wavelength region of 230–245 nm. The precursors used to generate Cl atoms were CH2Cl2, CF2Cl2 and CCl4. Two-photon-allowed transitions of HCl and DCl molecules were also observed in this region, with a detection sensitivity of better than 1010 molecules/cm3.

The detection of bromine atoms by resonant multiphoton ionization

Abstract Br(4p 2 P 3/2 o ) and Br * (4p 2 P 1/2 o ) atoms have been detected by resonant (2 + 1) multiphoton ionization, using tunable laser radiation between 240 and 285 nm. The bromine atoms were generated by the photolysis of Br 2 . HBr, CH 3 Br and C 2 H 5 Br. A total of 21 atomic transitions were detected, and in addition several intense molecular bands of Br 2 , HBr and CH 3 Br were observed.

The kinetic isotope effect in the reaction of O(3P) with H2, D2, and HD

The reaction of O(3P)+H2, D2, and HD was studied by the flash photolysis‐resonance fluorescence method. The rate constants kXY(T) were measured over the temperature ranges 297–471 K for XY=H2, 422–472 K for D2, and 422–473 K for HD. Above 400 K kH2 are in excellent agreement with earlier flow measurements, but below this temperature most of the flow data are systematically lower. A small deviation from the Arrhenius function was observed for kH2 at 297 K. Our measurements of kD2 are in excellent agreement with the results of Westenberg and deHaas and differ from those Clyne and Thrush. The measured values of kHD equal the arithmetic mean of kH2 and kD2 within experimental error. A detailed comparison is made between the present results and the CEQB calculations of Bowman, Wagner, Walch, and Dunning and the ICVT/LAG calculations of Garrett and Truhlar. Good overall agreement is obtained with both theories.

Evidence for tunneling in the reaction O(3P)+HD

Abstract The intramolecular kinetic isotope effect was determined for the title reaction by measuring laser-induced fluorescence from OH and OD in a discharge-flow apparatus. The OH/OD branching ratio was found to have a strong negative temperature dependence, showing that tunneling dominates the reaction at low temperatures. An apparent deviation from tunneling-corrected transition state theory calculations at low temperatures remains to be explained.

Optoacoustic measurements of IR multiphoton excitation of cis‐3, 4‐dichlorocyclobutene

Optoacoustic detection was used to study the infrared multiphoton excitation of cis‐3,4‐dichlorocyclobutene (DCCB). The dependence of the optoacoustic signal SOA on DCCB pressure PDCCB, on Ar buffer gas pressure PAr, and on laser fluence φ were studied systematically. It was observed that SOA is proportional to PDCCB and increases monotonically with PAr, leveling off at PAr∼10 Torr. Absolute calibration of SOA was achieved by measuring the attenuation of the laser beam. It was found that the average number of photons absorbed by DCCB n varies as 5.4 φ0.85, where φ is expressed in J/cm2. It is shown that the buffer gas pressure dependence of SOA can be used to deduce the vibrational relaxation rate of the excited molecule. The fluence dependence of n is predicted quantitatively by numerical solution of the master equation, including collision terms. It is assumed in this calculation that the infrared absorption cross section is independent of vibrational energy and is equal to the nominal small signal cr...

The intramolecular kinetic isotope effect for the reaction O(3P)+HD

The branching ratio for the reaction O(3P)+HD to produce OH and OD was measured over the temperature range 339–500 K using a discharge‐flow reactor. The OH and OD products were detected using laser‐induced fluorescence under steady‐state conditions. In order to determine the relative concentrations of OH and OD produced in this reaction, calibration runs were performed using the reactions of O(3P)+H2 and D2 to produce known relative amounts of the same species. Kinetic modeling showed that secondary reactions and the production of vibrationally excited OD did not affect the measured branching ratio. We found that the OH/OD ratio increased rapidly with decreasing temperature, in qualitative agreement with theory, showing that the reaction is dominated by tunneling below 400 K. At the lowest temperatures, the observed branching ratio is larger than predicted. The reason for this discrepancy is not known.

Vibrational relaxation of highly excited molecules: VV transfer from SF6 to N2O

Abstract Vibrational energy transfer from SF 6 to N 2 O was studied as a function of SF 6 vibrational energy. The intensity, rise time and decay time of N 2 O fluorescence increased monotonically with the level of donor excitation. The observations are consistent with a mechanism that is not mode specific, with donor VT relaxation faster than intermolecular VV transfer.

Infrared multiphoton induced isomerization of cis‐3,4‐ dichlorocyclobutene. I. Experimental results

The yield and product distribution in the laser‐induced isomerization of cis‐3,4‐dichlorocyclobutene (DCCB) have been measured as functions of laser fluence, DCCB pressure, and buffer gas pressure. Both Woodward–Hoffman allowed and ‘‘forbidden’’ isomers were observed. The fraction of ‘‘forbidden’’ products was found to increase linearly with fluence above a threshold of 3.5 J/cm2. At high fluence the yield increased quadratically with neat DCCB pressure. Both the yield and fraction of ‘‘forbidden’’ products declined monotonically with buffer gas pressure. These observations are explained in terms of a two‐channel mechanism, in which the allowed isomer is formed in a concerted path and the forbidden isomers are produced from a diradical intermediate in a nonconcerted reaction.