William M. Jackson

Laser‐induced fluorescence of CN(X2 Σ+) produced by photolysis of C2N2 at 160 nm

Laser‐induced fluorescence has been used to investigate the energy partitioning into the electronic, vibrational, rotational, and translational degrees of freedom of CN radicals produced in the photolysis of cyanogen (C2N2) at λ=160 nm (7.75 eV, 62 500 cm−1). The CN radicals that were produced in the (X2 Σ+) state were found to be vibrationally and rotationally excited. The initial distributions of the rotational line intensities of the (0,0) and (1,1) bands in the violet (B2 Σ+↔X2 Σ+) band system are characterized by Boltzmann distributions in which Trot (0,0) ∼1400 K and Trot(1,1) ∼1100 K. The observed Boltzmann character of the rotational state distributions is explained by a type II predissociation, which produces statistically randomized energy distributions in the fragments. It is observed that approximately 95% of the vibrational energy in CN(X2 Σ+) appears in the v″=0 and v″=1 levels. Evidence is presented which shows that C2N2 predissociates to produce equal amounts of CN(A2 Πi) and CN(X2 Σ+).

Laser measurements of the effects of vibrational energy on the reactions of CN

Pulsed laser photolysis of C2N2 at 193 nm has been used as a source of CN radicals in both the v″=0 and v″=1 levels. Individual rovibronic levels of these radicals were measured as a function of time with a tunable dye laser. From these measurements the rate constants for the reaction of each of these vibrational level with H2, O2, CO, CO2, N2, HCN, C2N2, and CH4 have been determined. Some enhancement in the rate constant with vibrational energy which could not be ascribed to quenching was observed for O2, CH4, and H2. Only vibrational quenching was observed for HCN, N2, CO2, CO, and C2N2. In the CO case the vibrational quenching rate appears to be significantly enhanced by complex formation during the quenching process.

Multiphoton ultraviolet photochemistry

Abstract Multiphoton photodissociation has been observed in C 2 N 2 , C 2 H 2 , C 2 H 4 , CH 3 OH, and C 2 H 5 OH. In all of these molecules fluorescence from small free radicals such as CN, CH, and OH has been observed. None of the observed fluorescence can result from the absorption of a single 193 nm photon. Arguments are presented which suggest that the observed results are best explained by invoking a sequential absorption scheme where the excited molecule absorbs a second laser photon rather than predissociating. Emissions have also been observed when H 2 O is photolyzed with the ArFlaser. In H 2 O the evidence suggest that this emission arises from collisional dissociation of two excited H 2 O molecules.

Coaxial measurement of the translational energy distribution of CS produced in the laser photolysis of CS2 at 193 nm

Carbon disulfide (CS2) photolysis was investigated in the gas phase using an argon fluoride (ArF) laser at 193 nm. The coaxial time‐of‐flight (TOF) distributions of CS radicals produced in the photolysis have been measured. Photochemical fragments have been observed with translational energies below 3 kcal/mol. The vibrational distribution of the CS fragments was also probed by laser induced fluorescence (LIF), and these measurements confirm that significant amounts of CS radicals are produced in vibrational levels greater than v‘=6. From a computer simulation of the experimental LIF data, a vibrational distribution was also obtained. Vibrational levels up to v‘=12 were found to be populated in a bimodal distribution, which peaks at v‘=4, and extends to v‘=12. There was a significant amount of rotational excitation of nascent CS produced in high vibrational levels of the ground state. The disjoint translational energy and CS vibrational energy distributions can be used to obtain an estimate of the S(3P) t...

A large vibrational enhancement in the reaction of CN(v‘=1, 2)+NO

A large vibrational enhancement of the rate constant has been observed in the two‐body reaction of CN(v″=1 and 2) with NO. NO also has a high efficiency as a third body in the termolecular recombination reaction between CN and NO. The rate constant obtained in this study for the bimolecular reaction between NO and CN (X 2 ∑ +) radicals in v″=0,1, and 2 are 1.6 ± 0.3×10−13, 5.6±0.8×10−11, and 6.2±1.0×10−11 cm3 molecule−1 s−1, respectively. The termolecular recombination reaction between CN(v″=0) and NO using Ar as a third body is 6.0±0.6×10−31 cm6  molecule−2 s−1, whereas when NO is the third body, the rate constant is 3.2±0.3×10−29 cm6 molecule−2 s−1.

The lifetime of the OH radical in comets at 1 AU

It has been shown that the photochemical lifetime of OH in comets is a function of the comets radial velocity. The calculated lifetime at 1 AU can vary between 69,000 and 210,000 sec for radial velocities that vary from -58 to +59 km/sec. A comparison between the scale lengths observed for three comets and those calculated based upon the theoretical lifetime has been made. This comparison shows that in two of the comets the lifetime derived from the scale lengths is a factor of 1.7 larger than the theoretical lifetime. Suggestions are made about the origin of this discrepancy.

Multiphoton ultraviolet photodissociation of C2N2

A detailed study of the multiphoton absorption of ArF laser photons in C2N2 has been done. Evidence is presented which shows that sequential two photon absorption occurs at threshold levels as low as 1024 photons/cm2 sec. A kinetic mechanism is proposed that agrees with both the observed pressure and intensity dependence of the fluorescence induced as a result of this sequential two photon absorption.

The dynamics of the C + NO reaction

Abstract The vacuum UV photolysis of C3O2 was used to produce C(3P) atoms. These atoms then react exothermically with NO and produce CN radicals. The quantum state distribution of the CN radical was determined using a tunable dye laser and the results are in agreement with the previously observed rate constant.

Vibronic effects in the predissociation of the C 1Πu state of C2N2

The partitioning of excess photochemical energy as a function of the vibrational energy of the C 1Πu state of C2N2 has been measured. Surprisal theory has been used to analyze the data and it shows that complete randomization does not occur before dissociation. The results are also inconsistent with the predictions of the quasidiatomic theory for photodissociation.