Vibrational excitation of hydrogen molecules by two-photon absorption and third-harmonic generation

Abstract

We report the coherent excitation of the vibrational state of hydrogen molecules by two-photon absorption and the resultant third-harmonic generation (THG). Parahydrogen molecules cooled by liquid nitrogen are irradiated by mid-infrared nanosecond pulses at 4.8 μm with a nearly Fourier-transform-limited linewidth. The first excited vibrational state of parahydrogen is populated by two-photon absorption of the mid-infrared photons. Because of the narrow linewidth of the mid-infrared pulses, coherence between the ground and excited states is sufficient to induce higher-order processes. Near-infrared photons from the THG are observed at 1.6 μm. The dependence of the intensity of the near-infrared radiation on mid-infrared pulse energy, target pressure, and cell length is determined. We used a simple formula for THG with consideration of realistic experimental conditions to explain the observed results.

Coherent two-photon emission from hydrogen molecules excited by counter-propagating laser pulses

“…The laser system is basically the same as the previous experiment where third-harmonic generation from the p-H 2 gas target was observed [39]. Here we describe the laser system briefly.…”

Geometry-dependent spectra and coherent-transient measurement of nearly degenerate four-wave mixing using two-photon resonance

“…The detailed laser setup is the same as that in Ref. [16,18]. The pump pulses are divided by a beam splitter into the pump1 and pump2 pulses.…”

“…The linewidth (FWHM) is roughly 150MHz, which is measured by absorption spectroscopy of the rovibrational transition of carbonyl sulfide gas. The detailed laser setup is the same as that in [18,20]. The pump pulses are divided by a beam splitter into those of pump1 and pump2.…”