Influence Of Permanent Dipole Moment Research Articles

Second harmonic generation at the quantum-interference induced transparency in semiconductor quantum wells: the influence of permanent dipole moments

The influence of permanent dipole moments of quantized states on intersubband second harmonic generation based on quantum-interference induced transparency in semiconductor quantum wells is explored using the harmonic balance method. The permanent moments are found to be quite important: they affect the transparency condition, especially at larger pump intensities. Hence, both the conversion efficiency and the optimal interaction path length change significantly when accounting for the permanent moments, and the conversion efficiency is reduced.

Dynamical quenching of laser-induced dissociations of heteronuclear diatomic molecules in intense infrared fields

This article explores the influence of permanent dipole moments, i.e., of direct vibrational excitations, on the dynamical dissociation quenching (DDQ) effect, a mechanism for laser-induced vibrational trapping in the infrared (IR) spectral range which was recently demonstrated for the homonuclear H2+ ion, and was shown to result from a proper synchronization of the molecular motions with the oscillations of the laser electric field [see F. Châteauneuf, T. Nguyen-Dang, N. Ouellet, and O. Atabek, J. Chem. Phys. 108, 3974 (1998)]. To this end, the wave packet dynamics of the HD+ and, to a lesser extent, the HCl+ molecular ions are considered in an intense IR laser field of variable frequency. Variations in the absolute phase of the laser electric field, a form of variations in the initial conditions, reveal new signatures of the DDQ effect due to the presence of nonzero permanent dipole moments in these molecules. The added permanent dipole/field interaction terms induce a discrimination between parallel and antiparallel configurations of the aligned molecule with respect to the laser’s instantaneous electric field. As a result, molecules that are prepared antiparallel to the field at peak intensity find their dissociation quenched most efficiently, while those that are prepared parallel to the field are strongly dissociative.

State-resolved collisional relaxation of highly vibrationally excited pyridine by CO2: Influence of a permanent dipole moment

The quenching of highly vibrationally excited pyridine through collisions with a 300 K bath of CO2 is investigated using high resolution transient infrared laser spectroscopy. Highly excited pyridine (Evib=37 950 cm−1) is prepared using pulsed ultraviolet (UV) excitation at 266 nm, followed by radiationless coupling to the ground electronic state. Energy gain into CO2 resulting from collisions with highly excited pyridine is probed using transient absorption techniques. Distributions of nascent CO2 rotational populations in both the ground (0000) state and the vibrationally excited (0001) state are determined from early time absorption measurements. Translational energy distributions of the recoiling CO2 in individual rovibrational states are determined through measurement of Doppler-broadened transient line shapes. These experiments investigate the influence of a large permanent dipole moment (μpyridine=2.2 D) on the collisional quenching dynamics of molecules with very large amounts of internal energy. A kinetic model is developed to describe rates for appearance of CO2 states resulting from collisions with excited pyridine as well as for quenching of excited pyridine at early times. These experiments show that collisions resulting in CO2 (0000) are accompanied by substantial excitation in rotation (Trot=1200 K for J=56–82) and translation (Ttrans=2900 K for J=78) while the vibrationally excited CO2 (0001) state has rotational and translational energy distributions near the initial 300 K distributions. Rate constants for the two energy transfer pathways are compared with previously published data on quenching collisions of excited (nonpolar) pyrazine, revealing only minor relative enhancement (∼2) in the vibrational excitation channel in pyridine relaxation. Overall quenching rates for excited pyridine are determined for both CO2 states investigated. These data show that the rotational and translational energy gain in CO2 is much more sensitive to collisional depletion of excited pyridine.

The influence of permanent dipole moments on second harmonic generation in asymmetric semiconductor quantum wells

The influence of permanent dipole moments on intersubband double resonance second harmonic generation in asymmetric quantum wells is analyzed. It is found that in realistic QWs matched for 10.6 μm pump the permanent dipole moments may have modest to significant influence on the conversion efficiency, especially at higher pump powers. At longer wavelengths, however, their influence increases, and should be taken into account in modeling the device performance.