David H. Parker

Photofragment imaging: The 266 nm photodissociation of CH3I

Abstract We use photofragment imaging to study the internal-state and velocity distributions of methyl fragments following photodissociation of CH 3 I molecules in a pulsed molecular beam by 266 nm radiation. The methyl fragments are state-selectively ionized via 2 + 1 resonance-enhanced multiphoton ionization (REMPI) through the 3p z Rydberg state. The velocity distribution for a particular internal state of the methyl radical is obtained from the images; this velocity distribution is then used to determine the branching of the methyl iodide into either the ground-state iodine, I( 2 P 3 2 ), or excited-state iodine I( 2 P 1 2 ), channel or the selected state of the methyl radical. We find that the branching ratio, I( 2 P 3 2 )/I( 2 P sol1 2 ), increases with increasing vibrational excitation in the methyl fragment. In addition, we use a line by line analysis to extract populations from the observed spectra of the 0 0 0 band of the 3p z ←X transition of the CH 3 fragment. The fit reproduces the observed spectrum and represents conservation of the K quantum number (spin about the C 3 axis) upon dissociation. For internally cold parent molecules, the amount of rotational energy about the fragment figure axis is found to be about 8 cm −1 and about 106 cm −1 for rotational energy perpendicular to the figure axis.

Mass-resolved laser ionization spectroscopy of HCl

Abstract We report 2 + 1 resonantly enhanced multiphoton ionization spectra of HCl between 82000 and 89000 cm −1 two-photon energy with mass resolution of the ions. We see significant production of atomic products subsequently ionized when the two-photonresonant state has a long bond length but we see only HCl + when the resonant state is of Rydberg origin (and hence has a short bond length). The state at 82780 cm −1 , assigned as the E 1 Σ + (ν = 0) state, exhibits the production of atoms that is characteristic of a state with a long bond length, evidence that this state should be incorporated into the double-minimum B 1 Σ + state.

Laser ionization spectroscopy of CD3 via the 3pz 2A‘2 Rydberg state

Nascent CD3 radicals produced by photodissociation of CD3I after absorption of 266 nm radiation by the A state are studied using the photofragment imaging technique. Two‐photon, resonance‐enhanced, multiphoton ionization [(2+1) REMPI] probes CD3 via the 3pz 2A’2 Rydberg excited electronic state. CD3I photodissociation at 266 nm is found to produce ground‐electronic‐state CD3 radicals with substantial vibrational excitation of the ν2 umbrella mode. Rotational constants are determined for v=0, 1, and 2 of ν2 in the 3pz excited state by analysis of the spectra. A first‐order perturbation and diagonalization procedure is used to generate potential energy curves for the umbrella mode in both the ground and excited electronic states and Franck–Condon factors for various transitions between the states. These results should prove useful when employing REMPI to characterize methyl radicals in many environments.

Direct measurement of rotational energy transfer rate constants for H35Cl (v=1)

We investigate the rotational energy transfer of H35Cl (v=1) in collisions with thermal HCl at 298 K. Rotationally state‐selective excitation of v=1 over J=1–6 is achieved by stimulated Raman pumping, and the rotational relaxation from the initially pumped level is monitored via 2+1 resonantly enhanced multiphoton ionization (REMPI) through the E 1Σ+–X 1Σ+0–1 band. The ions are detected in a time‐of‐flight mass spectrometer in order to ensure that only relaxation of H35Cl is observed. We present empirical correction factors for determining relative rotational populations from the REMPI spectral line intensities and extract the rate constants for rotational energy transfer from the time‐dependent populations using numerical techniques. The excellent sensitivity of the REMPI technique makes it possible to monitor the relaxation on very short collisional time scales (<0.1 hard sphere collision) and thereby enables us to determine the rate constants for both single quantum and multiquanta rotational transitio...

Saturation in laser-induced fluorescence: effect on alignment parameters

Abstract A method for extracting the alignment parameters of a molecular angular momentum distribution using laser-induced fluorescence is presented. The treatment is applicable to the common case of cylindrically symmetric orientation distributions in the high- J limit. Four different combinations of rotational branches in the LIF absorption emission process are examined for experimental arrangements without external fields as well as with an applied strong magnetic field. Computer algebra programs are used to generate simple analytical expressions which account for the influence of saturation on determining alignment parameters. In this analysis saturation effects can be calculated without any prior knowledge of the orientational distribution. It is found that an exact treatment is necessary even at a relatively low saturation and in most cases it can be carried out with only a limited knowledge of the saturation conditions.

Wavelength dependence of the BaO* product chemiluminescence on the N2O reactant orientation in the Ba + N2O → BaO* + N2 reaction

Abstract Coarse wavelength analysis of chemiluminescence from the crossed-beam reaction of Ba with oriented N 2 O reveals a distinct dependence of the BaO ★ final product internal energy distribution on the initial collision geometry. Favorable (Ba approaches the “O” end of N 2 O) and unfavorable (Ba approaches the “N” end of N 2 O) orientations are compared at two collision energies. Reactions taking place in the unfavorable orientation produce BaO ★ with relatively higher internal energy than reactions via the favorable orientation. The steric effect depends strongly on the wavelength and it is more pronounced at a higher collision energy.

Torsional mode relaxation of DABCO in a seeded supersonic beam

Abstract DABCOs ν 13 torsional mode relaxation is monitored in a helium-DABCO and argon-DABCO supersonic jet under low expansion conditions. Both cw and pulsed nozzles are employed. Modeling of the relaxation using the linear Landau-Teller relaxation equation is undertaken with various attempts to incorporate the effects of velocity slip. The relaxation rate is found to be independent of slip and the cross section dependent on the inverse of translational temperature. A V → R process is suggested as the rate determining mechanism.

Measurement of transition moments between molecular excited electronic states using the Autler‐Townes effect

This paper reports the observation of the Autler‐Townes splitting in the two‐color four‐photon ionization of hydrogen molecules, and the good agreement of theory with experiment. From the theoretical fits to the spectral profiles, the value of the electric dipole transition moment between the excited vibronic states E,F1Σ+g, v=6 and D1Πu, v’=2 is determined to be 2.0±0.5 a.u.

Saturation in Laser-Induced Fluorescence: Detection of Nascent Product Rotational Angular Momentum Alignment

Laser-induced-fluorescence (LDF) with polarization analysis is a powerful means of analysing angular momentum disposal in a chemical reaction. In many cases saturation effects can strongly perturb the information provided by the LIF signal on state populations and rotational angular momentum alignment. In this paper we extend a previous rate equation based treatment of LIF alignment analysis to cover most experimental configurations probing cylindrically symmetric anisotropic distributions produced by photodissociation and inelastic or reactive scattering processes.

Reactive Scattering with Oriented Molecules: Selectivity in the Ba + N 2 O → BaO* + N 2 Reaction

A series of oriented reactant studies of the chemiluminescent Ba + N2O → BaO* + N2 reaction are described. Hexapole fields state-select and focus single rovibrational levels of N2O for crossed beam reaction with Ba. The resulting chemiluminescence yield is measured for specified collision geometries, polarization, and coarse spectrum over a range of translational energies. Orientation dependent barriers to reaction are deduced using an angle-dependent line-of-centres model. An empirical correction for recrossing is discussed. Several features of the product channel are interpreted via the barrier to reaction.