Vibrationally excited benzene molecular cooling in supersonic beams

Abstract

There were examined photoionization spectra ofbenzene cooled in supersonic molecular beams. Compounds of benzene vapor with several buffer gases were used -He, Ne, Ar, Kr, N2, CH4. The photolonization was performed by frequency tuning KrF laser (linewidt 0.8 cm1; tuning range 120 cm-').The process is step-by-step, with the real intermediate level. The tuning range envelops two vibronic bands 6o lo 16,' and 6,I los, belonging to electron transition 'Aig Bu. The ionization spectrum is mainly depend on the absorption spectrum at the first step. The both bands are hot, but nevertheless the ionization efficiensy at supersonic beam conditions is rather high; it's caused by large "gap" of vibronic temperature from rotational and translative ones.As it was experimentally defined with various buffer gases, that bands intensity relation varies too. This fact testify about vibronic cooling dependence on the sort of buffer.Besids, a significant difference in the bands envelope contour widths was discovered. That is rather unusual fact, as the band width mainly depends on rotative temperature in the low state ( rotative band structure wasn't resolved in that experiments). Based on there data the rotative temperature estimation shows for benzene, that rotative temperature of molecules with excited 6th vibronic mode is always lower ( 4.5K with Ne buffer), than molecules with I 6t one ( I 5 K at the same conditions). This fact was interpreted as the rotationall-vibronic interaction result.As degenerated 6t mode has vibrational angular momentum, which is absent in the 16th, degenerated as well, so rotative-vibronic interaction in the first case s significantly stronge, and leads to levels splitting coparable with the rotative structure As the result rotative levels density for 6t mode twice more.The interraption moment for rotative relaxation, i.e the time, or characteristic distance from the nozzle cut-off, where rotative relaxation in the molecular beam forming on translative temperature and molecular density as well as sublevels encrg interval. The last is twice less for molecula with the excited 6 mode, that is the reasopn of the,r more intense rotative cooling.