Joseph I. Cline

Ion imaging measurement of collision-induced rotational alignment in Ar-NO scattering

Collision-induced rotational alignment of NO X 2Π1/2(v=0,j=8.5) is measured for rotationally inelastic scattering of NO X 2Π1/2(v=0,j=0.5) with Ar at 65 meV collision energy. The experiments are performed by velocity-mapped ion imaging with polarized 1+1′ REMPI probing of the scattered NO products. It is shown that the azimuthal information intrinsic to imaging detection allows the measurement of additional alignment moments not previously reported. The measured alignment shows only qualitative agreement with the predictions of the kinematic apse conservation model.

Photofragment μ‐v‐j correlation measured by 1+n′ resonance‐enhanced multiphoton ionization: Selective probing of bipolar moments and detection of chiral dynamics

A practical procedure is described to measure photofragment μ‐v‐j correlations using polarized 1+n′ resonance‐enhanced multiphoton ionization with a time‐of‐flight mass spectrometer detector. Following the theory of Dixon [R. N. Dixon, J. Chem. Phys. 85, 1866 (1986)], the correlations are expressed as the moments of a bipolar harmonic expansion of the correlated angular distribution of photofragment velocity and angular momentum (v and j) about the parent molecule transition dipole, μ. At a fixed detection geometry and on a single rotational transition, polarization control of the dissociating or probing light permits selective determination of targeted moments of the bipolar harmonic expansion. The velocity‐dependent spherical tensor moments of the angular momentum distribution depend upon these bipolar moments and are given for a general experimental geometry and for general elliptical polarization of the probing light. Several practical experimental geometries are described that isolate and measure tar...

Direct measurement of the binding energy of the NO dimer

The binding energy of the NO dimer has been measured directly using velocity-mapped ion imaging. NO dimer is photodissociated to produce NO(X) and NO(A), and the NO(A) is then nonresonantly ionized to NO+. The threshold for production of NO+ ions is measured at 44 893±2 cm−1, which corresponds to a binding energy of 696±4 cm−1.

Fluorescence and photodissociation of rhodamine 575 cations in a quadrupole ion trap

The fluorescence and photodissociation of rhodamine 575 cations confined to a quadrupole ion trap are observed during laser irradiation at 488 nm. The kinetics of photodissociation is measured by time-dependent mass spectra and time-dependent fluorescence. The rhodamine ion signal and fluorescence decay are studied as functions of buffer gas pressure, laser fluence, and irradiation time. The decay rates of the ions in the mass spectra agree with decay rates of the fluorescence. Some of the fragment ions also fluoresce and further dissociate. The photodissociation rate is found to depend on the incident laser fluence and buffer gas pressure. The implications of rapid absorption/fluorescence cycling for photodissociation of dye-labeled biomolecular ions under continuous irradiation are discussed.

Measurement of bipolar moments for photofragment angular correlations in ion imaging experiments

A general numerical method is given to extract angular correlations from photodissociation experiments with ion imaging detection. The angular correlations among the transition dipole moment of the parent molecule, μ, the photoproduct recoil velocity, v, and its angular momentum, j, are parametrized analytically using the semiclassical bipolar moment scheme due to Dixon. The method is a forward-convolution scheme which allows quantitative extraction of all measurable bipolar moments and can be applied in experiments with both linearly and circularly polarized probe light. It avoids the cylindrical symmetry limitations of the inverse Abel transform method, traditionally used for extracting photoproduct recoil anisotropy and speed distribution from imaging data. The method presented here also takes into account the possibility of multiple photodissociation channels. The features of the method are illustrated in a two-color 1+1′ REMPI-ion imaging study of the NO photoproduct trajectories resulting from the 6...

DETECTION OF ENDED NO RECOIL IN THE 355 NM NO2 PHOTODISSOCIATION MECHANISM

Circularly polarized 1+1′ REMPI with ion imaging detection was used to probe μ–v–j angular correlations of the NO (X 2Π1/2,3/2, v=0) photoproduct from linearly polarized 355 nm photodissociation of NO2 in a molecular beam. Rotational angular momentum orientation was measured for the NO product, with the sense and magnitude of the orientation depending upon the direction of the NO recoil velocity with respect to the polarization vector of the photolysis light. The rotational orientation is shown to originate from the directionality of the torque responsible for NO rotational motion. The experimentally observed sense of rotation in the plane of the NO2 parent molecule is consistent with an effective recoil impulse force applied to the N end of the NO product. A preliminary measure of the bipolar moment associated with this orientation for the j=21.5 and j=36.5 rotational levels gives a value of approximately β02(21)=0.2.

NO μ‐v‐j correlations in the photofragmentation of 2‐chloro‐2‐nitrosopropane

The trajectory of NO X 2ΠΩ=1/2,3/2(v=0) produced after excitation of 2‐chloro‐2‐nitrosopropane S1 A 1A″ is studied by polarized 1+1′ resonance‐enhanced multiphoton ionization with time‐of‐flight mass spectrometry detection. The correlations among the NO velocity v, angular momentum j, and the S1 A 1A″←S0 X 1A′ [n(N),π*(N=O)] transition dipole μ of the parent molecule are measured. The dissociation occurs by internal conversion to the ground S0 state or intersystem crossing to the T1 a 3A″ state and the observed speed distribution of the NO fragments is bimodal. There is no evidence for μ‐v‐j correlations for the products associated with the slow component of the speed distribution. For the higher speed component, the v‐j correlation is a function of the NO rotational state, j, and is described by the bipolar moment β00(22). The average value of β00(22) is −0.17(±0.02) at j≊11.5–22.5, more than five times larger than predicted by statistical phase space theory for dissociation on the S0 X 1A′ surface,...

Classical phase space theory for product state distributions with application to the v–j vector correlation

A classical phase space theory procedure for estimating product state distributions for ‘‘barrierless’’ dissociations is described. The distributions are determined in terms of an average over the available phase space on a fixed dividing surface of the function describing the distribution of interest. The use of a weighting function corresponding to the contribution of each randomly sampled phase space point to the number of available states, as in related state counting algorithms, provides an efficient route to the determination of any particular product state distribution or correlation. The coordinates employed in this sampling are the Euler angles, describing the orientation in space of each of the fragments and of the line of centers connecting the two fragments, and their conjugate momenta. Sample applications focus on the determination of the vector correlation between the angular momentum of one fragment and the relative velocity of the fragments for the dissociations of 2‐chloro‐2‐nitrosopropan...

Determination of μ-v-j vector correlations in photodissociation experiments using 2+n resonance-enhanced multiphoton ionization with time-of-flight mass spectrometer detection

A practical method is described for 2+n resonance-enhanced multiphoton-ionization probing of photofragment μ-v-j correlations arising in molecular photodissociation on a linearly polarized single-photon electric-dipole transition. The scheme uses polarized spectroscopy on the two-photon resonant transition with velocity detection by time-of-flight mass spectrometry. The technique is based on the theory of Kummel, Sitz, and Zare for polarized two-photon detection of angular momentum alignment and orientation and Dixon’s bipolar moment description of vector correlations. Optimal experimental and polarization geometries are described for selective measurement of targeted bipolar moments. The utility of the technique is demonstrated in experiments using 2+1 REMPI to probe methyl radical vector correlations in the 266 nm photodissociation of methyl iodide.

Photodissociation and collisional cooling of rhodamine 575 cations in a quadrupole ion trap.

The photodissociation of rhodamine 575 cations held in a quadrupole ion trap is studied using 514 nm light as a function of buffer gas pressure, irradiation time, and laser fluence. The laser-induced photodissociation decays of rhodamine ions have lifetimes on the order of seconds for the range of pressures and powers investigated and exhibit strong nonlinear pressure dependence. Dissociation mechanisms are considered that involve the sequential absorption of multiple photons and several collisional deactivation steps.