Effect of Laser Injection Seeder on Rotationally Resolved Spectra of Benzonitrile

High resolution lasers are necessary to derive the most information from molecular spectra. However, their use uncovers some photophysical processes that compromise the ability to resolve rotational structure. We study the influence of laser optical mode structure on the high resolution spectra of the S₁ states of benzonitrile in a supersonic molecular beam using an Ar⁺ pumped cw ring dye laser which is amplified by a pulsed Nd:YAG laser. The latter could be operated either in one optical mode by injecting (seeding) its oscillator with a single mode diode laser, or with many optical modes by not using the seeder. Rotationally resolved lines are obtained when the oscillator of the YAG laser are operated in one single optical mode, but only a continuum is seen when the YAG laser has multiple modes. It is argued that the ac Stark effect is the most probable reason for broadening and blurring the rotational lines.