Norio Morita

Time-delayed four-wave mixing using incoherent light for observation of ultrafast population relaxation

Time-delayed four-wave mixing using incoherent light has been proposed as a new method to measure the ultrafast population relaxation time T1. Assuming three input beams of incoherent light with much longer duration than T1, the theoretical calculation has shown that the decay curve by T1 appears in the signal profile and that the time resolution is determined by the correlation time of the light. The decay time to be observed is twice that of the transient four-wave mixing with short pulses. We have experimentally confirmed the prediction by observing picosecond population relaxations of organic dye solutions, using nanosecond pulses from a normal broadband dye laser. An advantage of the present method over the pump-probe method using incoherent light is also discussed.

Laser investigation of the competition between rotational autoionization and predissociation in superexcited np Rydberg states of NO

Rotational superexcited np Rydberg states (n≥23, v=0) of NO have been state‐selectively produced with a two‐color double resonance excitation method, and not only the NO+ ions generated by autoionization but also nitrogen atoms of the predissociation fragment have been directly detected to investigate the competition between rotational autoionization and predissociation. As a result, it has been found that the predissociation efficiency of the np Rydberg states hardly depends on whether the rotational autoionization channels are open or not. This fact clearly demonstrates that the decay dynamics of these states is governed by predissociation, not by rotational autoionization.

Rotational state dependence of decay dynamics in the superexcited 7f Rydberg state (υ=1) of NO

The decay dynamics of the 7f Rydberg state (υ=1) of NO has been investigated with laser multiphoton excitation methods; not only NO+ ions generated by autoionization, but also fragment nitrogen atoms produced by predissociation have been directly probed. The fragment atoms have been found to populate both the 2p3 2DJ and 2p3 4S3/2 states. Population yield in the 4S state shows strong dependence on each 7f rotational level of NO, and this dependence is remarkably correlated with autoionization yield, while no such significant dependence exists for the 2D state. From the fact that only odd L levels generate the 4S state, dissociative states causing this predissociation process have been identified as Σ+ states. Through detailed analysis, it has been shown that the predissociation rate of this channel is much larger than the autoionization rate, and that the N(2D)‐generating predissociation is also mainly caused by dissociative Σ+ states. Moreover, it has been found that a major part of the total decay rate ...

Three‐color triple resonance spectroscopy of highly excited ng Rydberg states of NO: Decay dynamics of high‐l Rydberg states

Author Institution: Department of Chemistry, Faculty of Science, Tohoku University; Department of Chemistry, Institute for Molecular Science

Laser Spectroscopic Investigation of High Rydberg States of no:Decay Dynamics Near the First Ionization Threshold

With a state-selective laser excitation, two main decay processes, autoionization and predissociation, in vibrational superexcited Rydberg states (n = 8–12, l = s, p, and f, v = 1) of NO have been studied, directly detecting not only NO

Infrared–infrared double resonance on a molecular beam of NH3

A novel laser spectroscopic method with a molecular beam is successfully demonstrated. This technique is widely applicable for studies of collisional relaxation on a variety of molecular transitions, even when untunable infrared molecular lasers are used. The effect of collision‐induced transitions between the vibration–rotation levels of NH3 are investigated by infrared–infrared double resonance. It is found that the rate of excitation energy transfer is closely related to the molecular energy level structure.

Double and triple resonance studies of rotational relaxation in NH3–He and NH3–H2 collisions

Collision‐induced transitions of NH3 in the collisions with He and H2 are studied by the laser‐microwave double and triple resonance methods. The results are summarized in the preference rules. It is confirmed that Δk=±3n transitions occur, while ortho–para conversion is still strictly forbidden.

Coherent propagation effect of incoherent light

The coherent propagation phenomenon of incoherent light through a resonant medium was experimentally demonstrated in the subpicosecond time region, using Na atoms and the light from an imperfectly mode-locked cw dye laser. The coherence spike with a 0.35-psec width in the cross-correlation profile between the propagated incoherent light and the initial light develops the same specific oscillatory structure as seen in the propagation of a weak coherent subpicosecond pulse. A theoretical analysis was also performed, and the experimental results are in considerably good agreement with the calculated ones.

Method for observing Doppler-free hot-band transitions

A novel spectroscopic method for highly sensitive detection of hot-band transitions based on infrared-infrared double resonance in a molecular beam was successfully demonstrated. The intensity of the infrared transitions from a vibrationally excited state is enhanced by two orders of magnitude by resonant laser pumping. The hot-band transitions are identified in connection with the assigned transitions in the fundamental band. Doppler-free spectral lines are observed. The experiment was carried out on the 2v3←v3 transitions of CH3F with CO2 laser pumping and probing.

Nonlinear Transient Spectroscopy with Ultrahigh Time-Resolution Using Light Sources with Controlled Coherence

The study of ultrafast phenomena is one of the major subjects of laser spectroscopy, for which numerous methods have been developed. The progress of ultrafast transient spectroscopy by means of picosecond and femtosecond pulse technology undoubtedly brought about revolutionary development in this field of research.1 There still remain, however, difficulties in the generation of practical light pulses and their applications in the time region far below 100 fs. The pulse broadening by material dispersion is one of fundamental limitations in all aspects of the technology associated with extremely short pulses. As another approach of studying ultrafast phenomena, various nonlinear spectroscopic methods in the frequency domain have early been developed.2–4 Although they served as complementary means to the time domain methods, they are still indirect and require careful interpretation.