A comparative study of the coherent Raman processes using the ruby and the second harmonic neodymium giant-pulsed lasers^*

¹ Department of Chemistry, University of California at Los Angeles, Los Angeles, California 90024.
² Electro-Optical Systems, Incorporated Subsidiary of Xerox Corporation, Pasadena, California 91107.

Abstract

Results will be reported of stimulated Raman scattering using a giant pulsed green laser (5 289 Å radiation) on a large number of organic liquids. Ten megawatts of green radiation were produced by doubling 1,06 μ neodymium laser output in a KDP crystal through precision index matching techniques. Similar Raman scattering experiments using the radiation from a giant-pulsed ruby laser (6 943 Å) provided results for critical comparison studies of major differences of the two types of scattering observations. These will be discussed in detail. The key results include :

1) Evidence that two-photon absorption is competitive with stimulated Raman scattering and is a quenching mechanism for the latter process when the laser frequency is approximately equal to one-half the frequency of an electronic absorption band of scattering sample. Moreover, some results on the inhibition of coherent Raman scattering are only explicable in terms of mixed two-photon (2ν_L — ν_R) + 2ν_L reactions, where ν_L and ν_R are the laser (1,06 μ) and Raman shifted frequency, respectively.

2) Indications that the 5 289 Å laser is more efficient for producing SRS with some substances than is the ruby laser. A tentative explanation in terms of the wavelength dependence of self-trapping of laser beams is proposed.

3) The stimulated Raman spectrum of benzonitrile contains shifts corresponding to two individual normal vibrations of the molecule and to sums and differences of these vibrations. Variations in the spectra obtained under different experimental conditions are discussed.

Résumé

On donne ici des résultats sur la diffusion Raman stimulée à l’aide d’un puise géant de laser vert (5 289 Å) sur un grand nombre de liquides organiques. On a obtenu une radiation verte de 10 mégawatts en doublant l’émission laser du néodyme de 1,06 μ dans un cristal KDP grâce à une adaptation précise des indices. Des expériences analogues de diffusion Raman en se servant de la radiation provenant d’un pulse géant de laser au rubis (6 943 Å) ont donné des résultats permettant de faire des études comparatives critiques des différences importantes existant entre les deux types d’observations de diffusion. Elles sont discutées en détail. Voici quels en sont les résultats marquants : il est prouvé qu’en compétition avec l’effet Raman stimulé, il se produit une absorption de deux photons qui est un mécanisme d’extinction pour ce processus quand la fréquence du laser est approximativement égale à la moitié de la fréquence d’une bande d'absorption électronique de l’échantillon diffusant.