A study of the 3p Rydberg transitions of the methylbenzenes by multiphoton ionization spectroscopy

Abstract

Two of the 3p Rydberg transitions of the methylbenzenes are seen in their multiphoton ionization spectra. These are assigned to states which include the 3pz orbital, having symmetries 1A1 and 1B2 in toluene and 1B1u and 1B2u in p-xylene. The origins of these states appear at 52839 and 54734 cm−1 in toluene. The 3s Rydberg transitions do not appear strongly in these spectra, indicating that the 3pz states have a longer lifetime with respect to radiationless transitions or dissociation.

Molecular Multiphoton Spectroscopy with Ionization Detection

“…In two photon transitions, however, these states become allowed and with ionization detection the valence states do not produce signal, so the 3s transitions become very prominent in the spectrum. Their prominence is probably due more to the absence of valence transitions than to allowedness, however, since two-photon forbidden 3p transitions show up just as well in p-xylene (25). There the 3s transition is very weak, prob ably due to coupling with isoenergetic valence states.…”

“…Unpublished studies in our laboratory have shown that ratios for the lB2 u +-+-lA l g MPI transition of benzene are very close to those observed in fluorescence excitation studies as long as the intensity is kept low enough that the spectra are completely resolved. We have also found that transitions to the 3p orbitals of the methylbenzenes were quite intensity dependent although qualitative difference still allowed state assignments to be made (25). Three-photon transitions have been studied in ammonia by Glownia et al (40) using polarization to good effect.…”

“…These states are parity forbidden in one photon spectra and often hidden by very intense nearby absorption. In some cases, such as the methylbenzenes (25), new 3p Rydberg states are seen as two-photon reso nances, even though they are parity forbidden in two photons, and the 3s transitions are weak, indicating a stronger coupling to nearby valence states.…”

Ηλεκτρονιακη Φασματοσκοπια Των Κυκλοκετονων Και Των Μεθυλοπαραγωγων Τους

“…Λόγω δε του μεγαλύτερου μεγέθους των τροχιακών Rydberg από τα τροχιακά σθένους, τα πρώτα είναι περισσότερο δεκτικά διαταραχών από το περιβάλλον. Έτσι οι οξείες μεταβάσεις Rydberg, όταν το μόριο βρεθεί υπό υψηλή πίεση αδρανούς αερίου της τάξης μερικών ατμοσφαιρών, φαρδαίνουν ασύμμετρα και μετατοπίζονται προς μεγαλύτερες συχνότητες (15)(16)(17). Από την άλλη πλευρά τα χαμηλότερα μέλη των σειρών Rydberg (n=3) είναι περισσότερο ευαίσθητα σε επιδράσεις που προκαλούν στο μόριο οι αντικαταστάτες, όπως οι ρίζες μεθυλίου, και σε αλληλεπιδράσεις με μοριακές καταστάσεις σθένους, οι οποίες είναι ικανές να εξαφανίσουν την παρατηρούμενη δομή Rydberg, γεγονός που διευκολύνεται και από τις πολύ μικρές συνήθως τιμές της ισχύος ταλαντωτή f των μεταβάσεων αυτών.…”