Photofragment Imaging Research Articles

An investigation of the 355 nm photodissociation of NO2 by state-resolved photofragment imaging

The 355 nm photodissociation of NO2 cooled in a supersonic beam has been investigated by state-resolved photofragment imaging. The NO and O(3PJ) photofragments were state-selectively ionized and projected onto a two-dimensional, position-sensitive detector to obtain speed and angular distributions. The speed distribution of the O(3P2) fragment displays two peaks corresponding to oxygen produced in coincidence with NO(υ=0) and NO(υ=1). The angular distributions for the O(3P2) and for the NO in several vibrational and rotational levels can be characterized by an anisotropy parameter of β=1.2±0.3. This value, while higher than that measured previously, is consistent with a dissociation lifetime on the order of 200–400 fs and with the colder rotational temperature of the current beam experiment. The rotational distributions of the NO product are found to be in good agreement with other recent measurements.

Two-photon dissociation of SO 2 in the ultraviolet region

Two-photon photodissociation of SO 2 through the B 1B 1 state at 286–309 nm has been studied by a perfect focusing mass spectroscopy and a photofragment imaging technique. Sulfur atoms were detected as photofragments by a resonance-enhanced multiphoton ionization technique. The lower limits of the anisotropy parameters for angular distributions are 0.42 ± 0.06 for S( 1D) and 0.50 ± 0.05 for S( 3P). The velocity distributions of the S( 1D and 3P) photofragments are analyzed assuming dissociation pathways: SO 2 + 2 ħω→S( 1D and 3P) + O 2(a 1Δ and X 3Σ − g). The two-photon excite the 8 eV region is assigned to be B 1B 1 with C 2v symmetry.

Photofragment imaging: the 205-nm photodissociation of CH 3Br and CD 3Br

The photofragment imaging technique is used to measure the velocity distributions of the Br( 2P 3 2 ) and Br*( 2P 1 2 ) atoms formed following laser dissociation of CH 3Br and CD 3Br at 205 nm. The speed and angular distributions are markedly different for the two bromine electronic states. The Br* images are anisotropic and have the appearance of a cos 2θ angular distribution indicative of a parallel transition. The Br ground-state images appear more isotropic but contain a distinct sin 2θ angular component. The speed distributions extracted from the Br* images are narrow compared with the Br speed distributions. This indicates that the sibling methyl fragments formed in the Br dissociation channel have greater internal excitation than the methyl fragments formed in the Br* dissociation channel. The results are compared with methyl iodide dissociation and the importance of surface crossings in the dissociation is discussed.

Two-dimensional imaging of state-selected photofragments: the 355 nm photolysis of NO 2

Two-dimensional photofragment imaging has been applied to the 355 nm photodissociation of NO 2 in a supersonic beam. The NO fragments are state-selectively ionized and projected onto a two-dimensional position-sensitive detector. The original velocity distribution is reconstructed by an Abel transform of the observed image of the fragments. The speed distribution of a single rovibrational state of NO consists of a single peak as expected from conservation of momentum and energy. The anisotropy parameter, β, of the NO photofragments is found to be 1.40±0.20, which is significantly larger than previously reported values measured with effusive molecular beams. This discrepancy is explained by the effect of the rotation of the parent molecule.

Two‐dimensional imaging of photofragments and species desorbed from surfaces: Energy and angle resolved studies

A two‐dimensional imaging method involving resonance‐enhanced multiphoton ionization to study the energy and the angular distributions of photofragments and photodesorbed species is described. Preliminary results obtained for NO adsorbed on Pt(111) and NO2 adsorbed on Al2O3(112̄0) with the technique are presented. The latter system has also been investigated by time‐of‐flight mass spectrometric analysis with excimer lasers. The results are discussed.

Photodissociation of acetylene: Determination of D00 (HCC–H) by photofragment imaging

Acetylene cooled in a He supersonic expansion is photodissociated by excitation in the 201–216 nm region of the à 1Au −X̃ 1∑+g transition. Subsequent ionization of the H-atom fragments by 2+1 (243 nm) REMPI, and mass-selected ion imaging allows analysis of the velocity distribution of H-atoms from the HCCH hν→ C2H+H process. Measurement of the maximum velocity for H atoms from this channel produced by photodissociation of acetylene through the à 1Au −X̃ 1∑+g V70K10, 110V40K10, 210V50K10 and V50K10 vibronic transitions gives a value for D00 (HCC–H) of 131±1 kcal/mol. Other channels producing hydrogen atoms (including HC2 hν→ C2+H and HCCHhν→ HCCH+ hν→ C2H++H) are detected at all photon fluxes used. These multiphoton channels produce hydrogen atoms with higher translational energy and therefore obscure measurement of the maximum velocity of H atoms produced by single-photon dissociation of acetylene. Reduction of photon flux by more than two orders of magnitude to ∼5×106 J/cm2 gives a background, multiphoton, H-atom intensity of ≤7% of the peak primary dissociation intensity. Because this multiphoton background limits the detectability of fast H atoms from single-photon dissociation of acetylene, the dissociation energy reported here is an upper limit. Calculations of potential rovibronic excitation of the C2H fragment are discussed.

Photofragment imaging: The 266 nm photodissociation of CH 3I

We use photofragment imaging to study the internal-state and velocity distributions of methyl fragments following photodissociation of CH 3I 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( 2P 3 2 ), or excited-state iodine I( 2P 1 2 ), channel or the selected state of the methyl radical. We find that the branching ratio, I( 2P 3 2 )/I( 2P 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 ←X̃ 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.

Laser ionization spectroscopy of CD3 via the 3p z 2A″2 Rydberg state

Nascent CD3 radicals produced by photodissociation of CD3I after absorption of 266 nm radiation by the à 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.

Two‐dimensional Imaging ofPhotofragments

The technique of photofragment imaging is described, and several examples of the power of the technique are presented. Two‐dimensional images of state‐selected photofragments from the photodissociations of CD3I and H2S illustrate how photofragment imaging reveals β parameters, brancing ratios, Doppler profiles and vector correlations. Comparisons are made with Doppler profiling and one‐dimensional time‐of‐flight techniques.