Direct observation of intramolecular vibrational energy redistribution under white light excitation. II. Fluorescence spectra of fluorobenzene derivatives by controlled electron impact

The low-resolution (resolution: 0.6 nm) fluorescence spectra of fluorobenzene derivatives, o-, m- and p-fluorotoluene (o-, m- and p-FT) were observed to investigate intramolecular vibrational energy redistribution (IVR) under controlled electron impact. The o- and m-FT spectra mainly consisted of unstructured emission from optically inactive states (bath modes) populated through IVR. The p-FT spectrum consisted of structured emission from optically active states and unstructured emission. The high-resolution (resolution: 0.15 nm) fluorescence spectrum of p-FT was measured to estimate the fraction of the structured emission intensity to the total emission intensity. The IVR rate of p-FT under electron impact excitation was faster than that under laser excitation. The fraction did not depend on the incident energy of electrons from 20 to 200 eV, and thus the IVR acceleration is not attributable to breakdown of the Born approximation.