Photoionization with coherent 118 nm vacuum ultraviolet light is applied to study kinetic energies of gramicidin S and ferulic acid molecules, laser desorbed from a ferulic acid matrix with 266 nm light. Analysis of the matrix photodecomposition products shows decarboxylation as a main photochemical pathway involved with desorption. Entrainment of the polypeptide by the matrix molecules is found under typical conditions. However, the velocity distributions are not particularly narrow, being at most borderline supersonic.Since the introduction' of matrix-assisted UV laser desorption in 1988 for desorbing and ionizing large organic molecules, the technique has been investigated by many research groups (for a review see Ref.2), but the desorption process itself and the role the matrix molecules play are not yet well understood. The first systematic experimental3 and theoretical4 studies describing dynamics of the desorption process were published only recently. Early data on the kinetic energy of laser desorbed tryptophan ions' and neutrals6 from pure trypotophan have also been available. However, little is known about the photochemical fate of the matrix molecules, which, in most cases to date, are relatively small aromatic organic acids that empirically have been found to be effective for producing mass spectra of polypeptides and proteins.', Their vaporization behaviour has been considered theoretically, but not the role their decomposition may play in de~orption.~ In this work, both the chemical identity of desorbed particles and their kinetic energies were obtained by photoionizing the desorbed neutrals above the surface (post-ionization) prior to time-of-flight (TOF) mass analysis. The kinetic energy is obtained from a separate time-of-flight measurement over the drift region between the surface and the ionizing laser beam; the time distribution is obtained by varying the time delay between desorption and ionizing laser pulses; velocity and kinetic energies are obtained by transforming the arrival time distribution. Single-photon ionization with 118 nm vacuum ultraviolet (VUV) light generally causes minimal fragmentation and is nonselective in the photoionization of organic molecules.' Thus we chose this technique to gain data on the kinetics of a polypeptide desorbing from ferulic acid as a matrix, irradiated with 266 nm light. EXPERIMENTALThe desorption is caused by illuminating the sample with 5 ns pulses of quadrupled Nd:YAG radiation at 266nm using a model GCR-3 laser (Spectra Physics) which was focused with a 300 mm focal-length quartz lens to a spot size of approximately 200 pm diameter, a value derived from burn marks on heat-sensitive paper.Author to whom correspondence should be addressed By angle tuning the doubling crystals and by inserting a stack of quartz glass windows into the beam, the energy per pulse delivered to the sample could be varied between 5 and 200 115.Photoionization is caused by 118 nm, 10.5 eV photons, which are the ninth harmonic of an additional Nd:YAG laser (Quanta-Ray, Model ...

A quartz crystal microbalance (QCM) is employed to determine the desorption yield of neutral particles in matrix-assisted laser desorption ionization. For ferulic acid, the matrix substance used, the QCM mass resolution of 1 ng corresponds to a detection limit of =loL2 particles. Sample exposure effects are studied at different fluences by measuring the total desorption yield after consecutive laser pulses fired on the same spot on the sample. The dependence of the total yield on the fluence is studied for different angles of incidence of the laser beam. Varying the distance between the QCM and the sample yields information about the angular spread of the desorbed neutral particles. Additional information about the fluence threshold for the production of charged particles is gained by direct electronic measurements of sample charging.

An imaging spectrometer with a 256 element InGaAs diode array was combined with a high throughput optical arrangement for recording high quality NIR spectra (824 nm to 1700 nm) of plastics from a distance of 25 cm within 6.3 milliseconds. The considered spectral region was assessed to be suitable for plastic identification.

Millimetre-sized crystals of 2,5-dihydroxybenzoic acid and 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid) were grown from a cytochrome clmatrix solution. Effects of sample exposure to UV laser irradiation on the matrix-assisted laser desorption ionization mass spectra are reported for such single crystals. The exposure was varied by irradiating the same sample spot with different numbers of UV-laser pulses. The ion yield and mass resolution were monitored as a function of the sample exposure. Further, the minimum deposited laser energy needed for protein ion production (the threshold energy) for single crystals was determined for incidence angles between 35" and 6 0 ' with respect to the target surface normal. The results were compared with those obtained for polycrystalline samples containing micrometre-sized crystals.

In the range of 83 500-89 200 cm -', the photoionization spectrum of HI is strongly influenced by autoionization of Rydberg series converging to the higher lying 2B,,2 component of the spin-orbit split 'H ionic ground state. The photoelectron yield spectrum, exhibiting the various autoionization processes, has been measured on a rotationally cooled sample ( T,,, -13 K), with a spectral resolution of the exciting vacuum ultraviolet radiation of 0.5 cm -'. Using a multichannel quantum-defect theory treatment explicitly including rotation, we calculate spinorbit and rotational autoionization fine structure in the spectrum. In order to find a qualitative agreement of theory and experiment, the Rydberg states of HI have to be described in terms of a transition in angular momentum coupling cases, going from Hund's case (c) to case (e).