Richard L. Dubs

A multiphoton ionization study of the photodissociation dynamics of the S2 state of CH3ONO

Two‐color (1+1) REMPI (resonantly enhanced multiphoton ionization) photoelectron spectroscopy is used to probe the NO photofragments produced by the UV photodissociation of methyl nitrite, i.e., CH_3ONO+hν→CH_3ONO∗(S_2)→CH_3O⋅(X)+NO(X, v, J). The photofragments are produced in their ground electronic states but with high rotational and translational energy. NO fragment angular distributions, rotational state distributions, and spatial alignment are determined by photoion and photoelectron detection. The initial state alignment is obtained by the CDAD (circularly dichroic angular distribution) technique for the first time. CDAD measurements for rotational levels with 35.5≤J≤46.5 result in alignment parameters at the classical high‐J limit of A^(2)_0 =−0.4. This alignment is consistent with an ‘‘impulsive’’ dissociation mechanism in which photofragment recoil along the CH_3O–NO bond imparts substantial rotational angular momentum to the NO molecule resulting in a high‐J state distribution and preferential rotation in the plane of dissociation. These measurements clearly establish the utility of the CDAD method for probing chemical processes in which spatial alignment plays an important role. Photoion angular distributions are used to probe correlations between the CH_3ONO transition dipole moment, NO fragment velocity, and angular momentum. These correlations reveal additional details of the photolysis mechanism.

Circular dichroism in photoelectron angular distributions as a probe of atomic and molecular alignment

In this paper we show that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe atomic and molecular alignment in the gas phase. Careful choice of photon (left or right circularly polarized) propagation and photoelectron collection directions breaks the cylindrical symmetry of the target, giving rise to dichroic effects. CDAD exists in the electric dipole approximation. We illustrate the sensitivity of CDAD to alignment by considering photoionization of the A 2Sigma+ state of NO. Most of the cases of alignment we consider are created by multiphoton absorption while the others, more general, might be created in fragmentation, desorption, etc. The alignment created by n-photon absorption quickly reaches a classical limit which is reflected in the CDAD spectrum. Finally, we show that CDAD is also a sensitive probe of gas phase atomic state alignment by considering photoionization of the 7P3/2 state of cesium created by single photon absorption from the ground state.

Atomic and molecular alignment from photoelectron angular distributions in (n+1) resonantly enhanced multiphoton ionization

Two distinct (n+1) REMPI techniques for obtaining the alignment of gas phase atoms and molecules from photoelectron angular distributions are presented. In both methods, the alignment is extracted from the angular distributions independently of the photoionization dynamics. The first method, which takes advantage of circular dichroism in the angular distributions (CDAD) has already been established experimentally as a useful probe of state alignment. The theory outlined in previous work is expanded here. The second method involves photoionization with light linearly polarized along the photoelectron collection direction and is presented here for the first time.

Circular dichroism in photoemission from oriented molecules at surfaces

Measurements of a new observable quantity in photoemission from oriented molecules are reported. Using circularly polarized radiation, the photoemission intensities for a special geometry were found to depend on photon helicity. The basic theoretical description of the effect is outlined, showing that it occurs for all spatially fixed molecules in the electric dipole approximation and does not depend on electron-spin effects. Experimental case studies for diatomics (CO, NO), benzene, and the “heavy” molecule CH_3I, all oriented by adsorption on Pd(111) or graphite (0001), reveal huge intensity asymmetries in all cases. Comparisons of these measured asymmetries are also made with the results of model calculations based on oriented CO, NiCO, and NO. The effect provides a sensitive probe of molecular orientation and of photoemission dynamics.

Circular dichroism in photoelectron angular distributions from two‐color (1+1) REMPI of NO

A detailed experimental and theoretical study of dichroic effects in photoelectron angular distributions is reported for (1+1), two‐color REMPI of NO via the A^ 2Σ^+, v=0 state. Optically aligned A state rotational levels are probed through ionization by circularly polarized light. Resultant photoelectron angular distributions exhibit significant left–right asymmetry, the phase and magnitude of which are shown to be related to the curvature of the excited state M_J distribution. Theoretical calculations involving a full ab initio treatment of the ionization dynamics result in circularly dichroic angular distribution (CDAD) parameters in good agreement with those derived experimentally. Additional effects including hyperfine depolarization and coherence are also discussed in relation to the observed CDAD data.

A multichannel quantum defect half‐collision analysis of K2 photodissociation through the B 1Πu state

Recent experiments by Zafiropulos et al. [Phys. Rev. Lett. 61, 1485 (1988)] indicate that K2 photodissociation through the B 1Πu state results in fluorescence polarization which is strongly dependent on excitation wavelength. To understand these results, we have studied the K2 system quantum mechanically using a half‐collision analysis derived from the generalized form of multichannel quantum defect theory. This analysis factors the transition amplitudes into separate terms representing absorption and final state interactions. An approximation called the adiabatic(a→c)/recoil(c→e) approximation is developed for the half‐collision matrix which reproduces quantitatively the exact half‐collision results projected from the close‐coupled wave function. This specific approximation applies to the homonuclear molecule K2 because of the very long range of the excited state potential, which varies as 1/R3. The quantum mechanical expression for the polarization as a function of initial rotational quantum number J0 a...

Intersystem crossing in collisions of aligned Ca(4s5p1P)+He: A half collision analysis using multichannel quantum defect theory

A half collision analysis of alignment effects on intersystem crossing in the collisions of Ca(4s5p 1P) with He has been performed using generalized multichannel quantum defect theory (MCQDT). The theory provides a rigorous analytical representation of the numerically exact closed‐coupled scattering wave functions. The half collision analysis results in a factorization of the full quantum collision problem into a number of simpler quantum mechanical problems which reflect different regions of development during the collision. A WKB‐assisted, frame transformation approximation to the incoming half collision matrix is tested numerically and is found to be useful in projecting out information on the ‘‘locking radius’’ concept.

Extraction of alignment parameters from circular dichroic photoelectron angular distribution (CDAD) measurements

In a previous paper, we showed that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe alignment in gas phase atoms and linear molecules. Often this alignment is parametrized through the moments of alignment A(2), A(4), etc., which are commonly extracted from fluorescence polarization measurements. In this paper we show how these can be simply extracted from CDAD spectra. This technique can be used in principle to extract the moments to any order.

(1+ 1) CDAD: A new technique for studying photofragment alignment

We report a new technique for measuring photofragment alignment in the gas phase by observing circular dichroism in photoelectron angular distributions (CDAD). This technique is well suited for determining the gas phase alignment of linear molecules. The experiment involves excitation of the photofragment with linearly polarized light followed by photoionization with left or right circularly polarized light. The difference between the photoelectron angular distributions for these two cases is the CDAD spectrum. By measuring CDAD through two different excitation branches, one can obtain the ground state photofragment alignment A (2) 0 using a simple analytical formula independent of the photoionization dynamics.

Cooper minima and circular dichroism in photoelectron angular distributions

We demonstrate that circular dichroism in photoelectron angular distributions (CDAD), resulting from resonance enhanced multiphoton ionization (REMPI) of an aligned molecular Rydberg state, is a highly sensitive probe of the presence of a Cooper minimum near threshold. To illustrate this application of CDAD, we present the results of ab initio calculations for (1+1’) REMPI via the R 21(5.5) branch of the D  ^2Σ^+(3pσ) state of NO, where a Cooper minimum is found in l=2 (d wave) of the kπ continuum at a photoelectron kinetic energy of 3.2 eV. The CDAD signal is found to vary rapidly with photoelectron kinetic energy, go through zero, and change sign in the region of the Cooper minimum. This result is predicted by CDAD theory for photoionization from an aligned atomic p orbital.