Ultrafast resonance-enhanced multiphoton ionization in the azabenzenes: Pyridine, pyridazine, pyrimidine, and pyrazine

Abstract

We report on the ultrafast photoionization of pyridine, pyridazine, pyrimidine, and pyrazine. These four molecules represent a systematic series of perturbations into the structure of a benzene ring which explores the substitution of a C-H entity with a nitrogen atom, creating a heterocyclic structure. Data are recorded under intense-field, single-molecule conditions. The pulses (50 fs, 800 nm) are focused into the molecular vapor, and ion mass spectra are recorded for intensities of ~10(13) W/cm(2) to ~10(15) W/cm(2). We measure ion yields in the absence of the focal volume effect without the need for deconvolution of the data. For all targets, stable singly- and doubly-charged parent ions (C(6-n)H(6-n)N(n)(+(+))) are observed with features suggesting resonance-enhanced ionization. From the intensity dependence of the ion yield, we infer that excitation occurs both through (1)ππ* transitions (remnants of the benzene structure) and through (1)nπ* transitions, the latter being a result of Rydberg-like excitations of the lone pair electrons of the nitrogen atoms. Stability against intense-field fragmentation is also discussed.

Analytical model for calibrating laser intensity in strong-field-ionization experiments

“…Here we investigate several aromatic molecules that have been reported in the literature. Uiterwaal and his group [66][67][68] have reported ionization of several organic molecules in a micrometer-sized interaction volume and measured the time of flight in an ion mass spectrometer using 50-fs, 800-nm laser pulses. The novel feature of their setup is that it allows the measurement of ionization yields without the need to carry out volume integration.…”

Imaging of alignment and structural changes of carbon disulfide molecules using ultrafast electron diffraction

“…Further experimental details may be found in Refs. [22][23][24][25]. The inverse Fourier transform of the diffraction pattern intensity I is the autocorrelation function of the molecular structure convolved with its angular distribution (see the Molecular images section in Methods).…”

Observation and Identification of Metastable Excited States in Ultrafast Laser-Ionized Pyridine

“…However, at present, no solid conclusions can be drawn in this regard. Pyridine was ionized with focused, high-intensity, 50 fs laser pulses at 800 nm, with (3 + 3) REMPI being the dominant ionization mechanism [23]. The data presented in Figure 2 differs from the REMPI experiment in that Figure 2 represents only 2-dimensional spatial resolution; the REMPI signatures are not visible here, but this more accurately describes the comparative yields of the parent and metastable ions.…”

“…The latter seems likely as it appears to resolve into a triplet peak shortly before extinction, similar to the (1), (2), (3) triplet. Our experiment is capable of producing multiply-charged ions [23], and the existence of the prompt multiply-charged parent ion is shown in Figure 1. Mass calibration of the TOF data in Figure 1 results in m/z=39.5 for this peak.…”