Time-of-flight mass spectrometry of aromatic molecules subjected to high intensity laser beams

Abstract

The recent introduction of femtosecond technology to pulsed lasers has led to the development of femtosecond laser mass spectrometry (FLMS). The present paper describes an FLMS investigation of the aromatic molecules, benzene, toluene and naphthalene. Wavelengths of 750 and 375 nm were used with beam intensities up to 4 × 1014 W cm−2. Pulse widths were of the order of 50–90 fs. The laser system was coupled to a linear time-of-flight mass spectrometer. This experimental method of chemical analysis is gaining momentum, often replacing its nanosecond forerunner, resonant enhanced multiphoton ionization. For the said molecules, predominant parent ion production is found, making identification unambiguous. In fact this characteristic is being consistently attained in small to medium mass molecules irradiated under similar conditions, leading to the conclusion that a universal chemical detection system is a possibility. Such soft ionization is particularly evident at longer wavelengths (∼750 nm) with less relative fragmentation, daughter ion formation, compared to results at shorter wavelengths (∼375 nm). In terms of parent ion formation, similar numbers are produced with laser intensities around 1014 W cm−2 for both wavelengths. It has also been shown that at a threshold of about 5 × 1013 W cm−2, double ionized molecules appear for the 750 nm wavelength. These interesting new mass spectra display intense single, double and even triple ionized peaks without significantly increased dissociation. Such effects are less pronounced at 375 nm. © 1998 John Wiley & Sons, Ltd.