Photoelectron spectroscopic evidence for the competition of ionization and internal conversion at the S2 excited state of trimethylamine

Abstract

Laser photoelectron energy measurements in a molecular beam of trimethylamine were carried out at the laser wavelengths 192.1, 208.8, 228.7, and 252.7 nm. The photoelectron spectra observed at 192.1 and 208.8 nm can be attributed to two ionization processes competing with each other: one from the optically prepared S2 state and the other from the vibrationally excited SI state (S,t) produced from the internal conversion S2* --SI$. IntroductionLaser excited-state photoelectron spectroscopy with a resonant two-or multiphoton ionization technique is very useful for characterization of various electronically excited states of molecules in the gas phase.' This technique makes it possible to provide information about excited states which cannot be detected by optical emission spectroscopy.When photoionization occurs at a specific excited state in competition with some intramolecular relaxation such as internal conversion and intramolecular vibrational redistribution, photoelectrons can be ejected from relaxed states as well as from the optically prepared excited state under appropriate laser irradiating conditions. The molecule in the initially prepared state is either ionized directly with the rate constant lai ( I is the laser fluence and u, is the ionization cross section) or relaxed to another state with the rate constant k (which is a molecular constant) and then

Critical Landau Velocity in Helium Nanodroplets

“…The lowest excited singlet states of these molecules are of Rydberg character with the 2p electron of the nitrogen atom excited to the 3s or the 3p orbital [35][36][37][38]. Excitation to these states results in the ejection of the molecules from the helium droplets, as has been explicitly checked for by photoelectron spectroscopy.…”

Electronic Spectroscopy and Ultrafast Energy Relaxation Pathways in the Lowest Rydberg States of Trimethylamine

“…They all feature pyramidal ground electronic states and planar Rydberg excited states, and they have internal rotations associated with the methyl groups 2. The smallest tertiary amine, trimethylamine (TMA), N(CH 3 ) 3 , has been used as a prototype to explore the spectroscopy and photochemistry of this class of compounds 9,10,11. Among the amines, the tertiary amines have the lowest ionization energies,12 causing the Rydberg series to start at a relatively low energy, about 4.5 eV.…”

“…In past work, photoionization photoelectron spectroscopy experiments have been performed on trimethylamine using nanosecond laser pulses. 10,31 That time scale is poorly matched to the molecular dynamics because the molecules initially prepared in 3p convert to 3s within the duration of the nanosecond pulse, so that ionization always proceeds out of the latter. Our experiments were performed with pulses of femtosecond and picosecond duration.…”

“…They all feature pyramidal ground electronic states and planar Rydberg excited states, and they have internal rotations associated with the methyl groups . The smallest tertiary amine, trimethylamine (TMA), N(CH 3 ) 3 , has been used as a prototype to explore the spectroscopy and photochemistry of this class of compounds. − Among the amines, the tertiary amines have the lowest ionization energies, causing the Rydberg series to start at a relatively low energy, about 4.5 eV. At excitation wavelengths below 220 nm, TMA has a very low fluorescence quantum yield, and following excitation at 193 nm it converts to the ground state surface and dissociates a methyl radical to form the N(CH 3 ) 2 product. , …”

“…The lifetimes of the 3p states of trimethylamine are short but not so short that spectral separation of the 3p states is impossible. In past work, photoionization photoelectron spectroscopy experiments have been performed on trimethylamine using nanosecond laser pulses. , That time scale is poorly matched to the molecular dynamics because the molecules initially prepared in 3p convert to 3s within the duration of the nanosecond pulse, so that ionization always proceeds out of the latter. Our experiments were performed with pulses of femtosecond and picosecond duration.…”