Imaging of supersonically expanded and pulsed molecular beams by nonlinear photoionization processes

In molecular spectroscopy, imaging of supersonically expanded and jet-cooled molecular pulses to evaluate their time of flight and velocity is useful in obtaining photoionization spectra of higher resolution, well-resolved, and enhanced signal to noise ratio. In this paper, the ultraviolet output of frequency doubled OPO laser at 266 nm was employed to obtain highly-resolved (2+1) resonance-enhanced multiphoton ionization (REMPI) spectra for cooled molecular pulses of methyl iodide (CH₃I) sample seeded in helium gas. The recorded photoionization spectra were manipulated to study the shape, duration and structure of the jet cooled molecular beam pulses. A two meter time- of- flight (TOF) mass spectrometer was employed to identify and record the ions produced while varying the time delay between molecular pulses and laser shots. Imaging of CH₃I molecular pulses yielded lorentzian distributions of full width at base and flight time peak location corresponding to pulses duration of ∼ 0.3 milliseconds and 931 m/s cooled molecules translational velocity, respectively.