Intense-Field Photoionization of Molecules using Ultrashort Radiation Pulses: REMPI in Toluene, Aniline, Phenol, and Fluorobenzene

We investigate the intense-field photodynamics of molecules using 50-fs pulses with intensities of 10¹² to 10¹⁴ W/cm² at 800-nm from a Ti:sapphire laser. We use a mass spectrometer that avoids focal intensity averaging. We have investigated the isoelectronic target molecules toluene (C₆H₅-CH₃), aniline (C₆H₅-NH₂), phenol (C₆H₅-OH), and fluorobenzene (C₆H₅-F). We discuss a resonance-enhanced multiphoton ionization (REMPI) mechanism that includes the AC Stark shifts of the molecular states. We have also investigated CO₂ and CS₂. For these molecules, no resonance effects are observed. (paper)