The rotating wave approximation, including the incorporation and importance of diagonal dipole moment matrix elements, for infrared multiphoton excitations

On the effects of permanent molecular dipoles in the simultaneous absorption of two photons: full generalized rotating wave approximation versus analytical results

The effects of permanent dipoles, and the relative effects of the direct permanent dipole and the virtual state excitation mechanisms, are discussed for excitations involving the simultaneous absorption of two identical photons. Two molecular models for two-photon excitation, one dominated by the direct permanent dipole mechanism and the other having significant contributions from both excitation mechanisms, are used for this purpose. Resonance profiles, as a function of laser intensity, are evaluated for both models by employing the full Generalized Rotating Wave Approximation method and the recently developed Analytic Generalized Rotating Wave Approximation (AGRWA). The profiles are used to assess (1) the nature of the effects of permanent molecular dipoles, (2) the relative contributions of the two excitation mechanisms, and (3) the validity of the AGRWA for two-photon excitations. The AGRWA is a very useful interpretive∕predictive tool even for higher laser intensities where its validity becomes questionable. It can be used to suggest how to exploit the effects of molecular permanent dipoles to enhance two photon excitations using both excitation mechanisms.

Electromagnetically induced transparency in a Λ-type molecular system with permanent dipole moments revisited

Electromagnetically induced transparency (EIT) in a molecular three-level Λ system with permanent dipole moments and undergoing m- and n-photon transitions by pump and probe lasers is investigated. Analytical expressions are derived for probe absorption spectrum and dispersion for a medium of stationary as well as thermal molecules. Contrary to the earlier study by Zhou et al. [J. Chem. Phys. 131, 034105 (2009)], we observe no amplification in 2 + 2 photon process when the sign of the difference of the permanent moments of the excited and the ground levels is reversed. Reasons for these contrasting observations are discussed. Our study shows that the permanent moments essentially damp the laser-molecule Rabi frequency to result in narrower EIT line width and larger group velocity index. These effects are further enhanced when the order of the multi-photon process is increased. The importance of the virtual mechanism is discussed by considering the special case of 2 + 1 photon EIT.

Electromagnetically induced transparency in a three-level lambda system with permanent dipole moments

Electromagnetically induced transparency in a three-level Lambda-type molecular system with nonzero permanent dipole moments is investigated. It is shown that in the (2+2)-transition processes, when the sign of d(21), the difference in permanent dipole moments of the probe transition, is positive, perfect electromagnetically induced transparency with steep normal dispersion could be obtained under specific conditions. In contrast, when the sign of d(21) is negative, surprisingly gain without inversion with steep anomalous dispersion could be attained.

Phase-sensitive stimulated Raman adiabatic passage in dipolar extended lambda systems

The authors investigate the possible phase-sensitive behavior of (multiphoton) stimulated Raman adiabatic passage population transfer in extended lambda systems, if more than one state of an anharmonic progression of target levels is accessible in transitions of different photonicities. They use a minimal model four-level system (4LS) with one initial state separated from two target states by an apex state. The parameters of the 4LS are adapted from the bend states of the HCN-HNC system. Using a dressed-state analysis within the rotating wave approximation (RWA), the authors identify phase-dependent diabatic transitions between the two dressed states contributing to the state vector as the mechanism leading to phase-sensitive target populations. The essential features giving rise to the phase dependence are found to be different (non-zero-) diagonal elements of the dipole matrix, i.e., permanent dipole moments, and the presence of a direct two-photon overtone coupling between the apex state and the lower target state which formally enters the RWA Hamiltonian upon inclusion of permanent dipole moments. Among the parameters controlling the extent of the effect are the anharmonic properties of the target progression and the absolute values of the field frequencies, so that in view of the requirement to tune the driving fields into the vicinity of resonance, details of the level structure are of importance. A comparative numerical study executed without invoking RWA shows that qualitatively there are similar trends in the appearance of phase sensitivity, although the effects are considerably more pronounced in the full treatment. In the full treatment the authors also explore off-resonance conditions and discuss the signatures of phase sensitivity in the target populations.

Non-Born–Oppenheimer calculations of the pure vibrational spectrum of HeH+

Very accurate calculations of the pure vibrational spectrum of the HeH(+) ion are reported. The method used does not assume the Born-Oppenheimer approximation, and the motion of both the electrons and the nuclei are treated on equal footing. In such an approach the vibrational motion cannot be decoupled from the motion of electrons, and thus the pure vibrational states are calculated as the states of the system with zero total angular momentum. The wave functions of the states are expanded in terms of explicitly correlated Gaussian basis functions multipled by even powers of the internuclear distance. The calculations yielded twelve bound states and corresponding eleven transition energies. Those are compared with the pure vibrational transition energies extracted from the experimental rovibrational spectrum.

The Electronic Spectrum of Gaseous CoO in the Visible Region

Laser excitation spectra of gaseous CoO have been recorded using Doppler-limited intracavity spectroscopy and supersonic jet-cooled molecular beam methods. As seen in the molecular beam spectra there are nearly 100 bands arising from the X4Delta7/2 spin component of the ground state in the wavelength region 430-720 nm. All of them are very strongly red-degraded and most of them are perturbed. 59Co hyperfine broadenings or splittings have been observed in many. The bands can be arranged into five electronic transitions, together with a considerable number of "extra" bands induced by perturbations; two of the excited electronic states are 4Phi, while the other three, which lie within 1700 cm-1, are 4Delta. Semi-empirical calculations have been carried out for the 4Delta excited states, using exchange integrals transferred from the TiO and VO spectra; the predicted energies, spin-orbit structures, and relative intensities in absorption are consistent with the assignment of all three 4Delta states to the same electron configuration, (4ssigma)1(3ddelta)3(3dpi)2(3dsigma)1. It seems that there are extensive interactions between the excited 4Delta states, because only one of them gives rise to a recognizable, though irregular, long vibrational progression in absorption. The other two progressions die out abruptly and unexpectedly after three or four members, presumably as a result of interference effects; however, wavelength-resolved fluorescence studies, together with calculations of the Franck-Condon overlap integrals to the various vibrational levels of the ground state, show that the upper state progressions reappear following a region of confused absorption. All of the excited states show highly irregular variations of their vibrational intervals and rotational constants with the vibrational quantum number, v. The wavelength-resolved fluorescence spectra give evidence for new low-lying electronic states of CoO at 3038, 5989, and 9105 cm-1; based on its vibrational frequency the 3038 cm-1 state is possibly the sigmadelta4pi2 B4Sigma- state. Copyright 1997 Academic Press. Copyright 1997Academic Press