Elizabeth McCormack

Rotational state distributions from vibrational autoionization of H2 revisited

Photoelectron spectra of vibrationally autoionized (X 2Σ+g)np, v=1,2 Rydberg states of H2 were obtained by using an optical–optical double‐resonance technique. The spectra were obtained by using a hemispherical electrostatic electron energy analyzer that had sufficient resolution to determine the rotational state distributions of the H+2 ions. The ionization process occurred in a magnetic and electric field‐free region. All of the results are consistent with the assumptions that the photoelectrons are ejected as p waves and that spin effects are negligible. This is in sharp contrast to our earlier results obtained by using a magnetic bottle electron spectrometer, in which the ionization process occurred in a 0.5–1.0 T magnetic field [J. L. Dehmer et al., J. Chem. Phys. 90, 6243 (1989)]. It is concluded that the magnetic field can significantly modify the rotational distributions observed for vibrational autoionization. The implications of this conclusion are discussed, with particular attention to other e...

Multiple Spacecraft Michelson Stellar Interferometer

An alternative to a monolithic space Michelson interferometer is a device composed of three separate spacecraft. Such a device consists of two telescopes which collect light from a source and transmit it to a third spacecraft positioned so that interference fringes are detectable in an onboard interferometer. The contrast of the fringes is then measured as a function of separation of the telescopes. A multiple spacecraft design allows extremely large interferometer baselines. The resulting high angular resolution permits fundamental astrophysical measurements different from those possible with foreseeable monolithic devices. We describe an orbital analysis and assessment of performance for a particular device design, SAMSI, Spacecraft Array for Michelson Spatial Interferometry. The device we consider includes two one-meter telescopes in orbits which are identical except for slightly differing inclinations. The telescopes achieve separations as large as 10 kilometers and relay starlight to a central station which has a one-meter optical delay line in one interferometer arm. Our four key findings are as follows: 1) a 1000 kilometer altitude, zero mean inclination orbit affords natural scanning of the 10 km baseline with departures from optical pathlength equality which are well within the corrective capacity of the optical delay line; 2) electric propulsion is completely adequate to provide the required spacecraft motions (principally those needed for repointing); 3) all necessary technology is already in a high state of development; and 4) resolution and magnitude limits of 10-5 arcsecond and my = 15 to 20 are achievable.

Double-resonance laser-induced grating spectroscopy of nitric oxide

Abstract Double-resonance laser-induced grating spectroscopy (LIGS), including two-color laser-induced grating spectroscopy (TC-LIGS) and degenerate four-wave mixing (DFWM) spectroscopy, has been used to probe the E 2Σ, ν=0←A 2Σ, ν′=0←X 2Π 1 2 , ν″=0 double-resonance transitions in NO. Several different double-resonance schemes have been implemented and their relative merits are discussed. The results demonstrate that double-resonance LIGS shows considerable promise for obtaining state-selective absorption spectra of rovibronically excited states for a wide variety of molecules.

Dissociation dynamics of high‐v Rydberg states of molecular hydrogen

A high‐resolution, state‐selective investigation of the decay of (X 2Σg+)np, v’≥9 Rydberg states of H2 into the H(1s)+H(3l) and H(1s)+H(4l) dissociation continua has been performed. The (X 2Σg+)np Rydberg states were excited by double‐resonance excitation via the E,F 1Σg+, v=6, J=0 and 1 states, and the H(3l) and H(4l) dissociation products were detected by monitoring the energy‐resolved photoelectrons produced by nonresonant photoionization of the fragments. The energy region of interest (138 800–140 000 cm−1) includes the X 2Σg+, v+=9, N+=0–3 ionization thresholds, the H(1s)+H(4l) dissociation threshold, and the H++H− ion‐pair threshold. Evidence of both direct and indirect dissociation involving both singly and doubly excited electronic states is observed. Comparisons to the previously observed X 2Σg+, v+=8 constant‐ionic‐state photoelectron spectrum and the ion‐pair spectrum observed in the same energy region reveal strong competition between the ionization and dissociation processes through both rovi...

Observation of hyperfine quantum beats in two-color laser-induced grating spectroscopy of nitric oxide

Abstract Two-color laser-induced grating spectroscopy of NO via the A 2 Σ + , v ′=0 state has been used to study the time evolution of population gratings produced in a supersonic molecular beam. For a specific polarization configuration of the input laser beams, quantum beats due to the hyperfine structure of the A 2 Σ + , v ′=0 state have been observed by varying the delay between the grating-forming laser pulses and the grating-probing laser pulse. This form of quantum beat spectroscopy has the advantages of background-free detection, two-color state selectivity, a quadratic dependence on the level population, and a beat signal independent of the decay mode.

Dynamics of Rydberg States of Nitric Oxide Probed By Two-Color Resonant Four-Wave Mixing Spectroscopy

Two-color resonant four-wave mixing (TC-RFWM) spectroscopy has been used to probe highly excited v=0 and v=1 Rydberg states of nitric oxide. Transitions to n=16–30, v=0, Rydberg states, and the 8p, 9p, 7f, 8f, 8s, and 9s, v=1 Rydberg states from the A 2Σ+, v′=0 and 1 states have been recorded. The decay rate of the 8p and 9p, v=1 states has been extracted from the observed line profiles by using a recently developed model for the excitation of quasibound resonances in TC-RFWM spectroscopy. Transitions from the A 2Σ+, v′=1 state to the X 2Π3/2, v″=10 state have also been observed, allowing an absolute calibration of the TC-RFWM signal intensity. This calibration is used to determine an excited-state absorption cross section for the 9p, v=1 Rydberg state.

Alignment of the E,F 1Σ+g, v’E=1 state of H2 by two‐photon excitation

The alignment of the E,F 1Σ+g, vE=1 state of H2 produced by two‐photon excitation from the X 1Σ+g, v‘=0 ground state was investigated using two different techniques. First, in a single‐color experiment, photoelectron angular distributions were measured for the two‐photon resonant, three‐photon ionization of H2 via the Q(0) and Q(1) transitions to the E,F 1Σ+g, v’E=1 state. The photoelectron angular distributions are consistent with an unaligned E,F 1Σ+g, vE=1, J’=1 state. Second, in a two‐color experiment, the photoionization spectra of several vibrationally autoionizing (X 2Σ+g)np, v=1 Rydberg states excited from the v’E=1, J’=1 level of the E,F 1Σ+g state were measured as a function of the relative polarizations of the pump and probe beams. The polarization dependence of the relative intensities of the P(1) and Q(1) transitions was used to determine the alignment of the E,F 1Σ+g, vE=1, J’=1 state produced by the pump laser. Consistent with the angular distribution measurements, the polarization results ...

MULTISTATE INTERACTIONS IN NITRIC OXIDE PROBED BY LASER-INDUCED GRATING SPECTROSCOPY

Two‐color laser‐induced grating spectroscopy (TC‐LIGS) via the A 2Σ+, v’=0 state has been used to study the B 2Π, v=28 and L 2Π, v=8 valence states and the Q 2Π, v=0 5pπ Rydberg state of NO. Energies for previously unobserved or unresolved transitions to the perturbed L 2Π1/2,3/2, v=8 and Q 2Π1/2,3/2, v=0 states are reported for the first time. The interaction between these states is reconsidered in light of the new assignments. A comparison of different detection methods made for the B 2Π1/2,3/2, v=28←A 2Σ+, v’=0 transitions demonstrates the ability of TC‐LIGS to detect states that are difficult to detect by ionization or fluorescence‐dip detection.

State-selective quantum beat spectroscopy via coherent control of Liouville-pathway interference in two-colour resonant four-wave mixing

Diagrammatic perturbation theory and the spherical tensor formalism are used to model the signal observed in time-resolved, two-colour resonant four-wave mixing (TC-RFWM). An analysis using the Liouville formalism illustrates how TC-RFWM can be used to perform state-selective quantum beat spectroscopy in three-level systems by suitably designing three experimental features: the excitation scheme for the matter-field interaction, the time ordering of the laser pulses and the polarization of the incident laser beams. In contrast to traditional fluorescence quantum beat spectroscopy, time-resolved TC-RFWM results in a background-free, coherent signal that allows for high-resolution detection of the spectrally unresolved energy structure within a selected material state of the three-level system. The beats are shown to result from the quantum interference between different Liouville-space pathways in the laser-induced evolution of the material density operator and appear as a temporal modulation in the amplitude of the light beam emerging as the output of the mixing process. This type of control of a coherent optical field, based on the selective preparation of superposition states, is reminiscent of other recently developed optical techniques such as coherent population trapping and lasing without inversion.

The effect of laser bandwidth on the signal detected in two-color, resonant four-wave mixing spectroscopy

The effect of laser line shape and bandwidth on the signal detected in two-color, resonant four-wave mixing (TC-RFWM) spectroscopy is determined by means of an ab initio calculation of the third-order polarization based on diagrammatic perturbation theory. Modifications to the approach previously used for the case of δ-function laser line shapes are made by introducing a different treatment of the rotating wave approximation and phase-matching conditions. A three-level excitation scheme for double-resonance spectroscopy of bound and quasibound states is analyzed. In the case of Lorentzian laser line shapes, analytic expressions for the signal line profile are obtained for each excitation scheme. Analytic approximations of the signal line profile are also obtained in the case of Gaussian laser line shapes.