VELOCITY DISTRIBUTIONS FOR A LASER-VAPORIZED, ANTHRACENE-LABELED NUCLEOTIDE

We report the laser vaporization and multiphoton ionization of two ant hracene-labeled molecules, N-(6-hydroxyhexyl)-3-(9-anthryl)propionamid e and an anthracene-linked deoxythymine monophosphate. Laser vaporizat ion of the anthracene-containing molecules was performed by directing, a high-intensity pulse (30-300 mJ/cm(2), 2.5 ns) of 532 nm laser ligh t into a thin film of rhodamine 6G (Rh 6G) containing nanomolar quanti ties of the sample. Resonance-enhanced multiphoton ionization was used to selectively ionize the anthracene moiety in each molecule after va porization. The S-0-S-1 transition of the anthracene molecule at 361 n m was used to excite the molecules, and ionization from this intermedi ate state was achieved using a 308 nm photon. The ionized products wer e detected and analyzed by mass spectrometry. The time-of-arrival meas urements for the laser-vaporized molecules correspond to a bimodal vel ocity distribution, composed of a thermal component and a hyperthermal component showing substantial translational energy cooling. The distr ibutions for the anthracene-labeled molecules reveal lower most probab le velocities than those for the Rh 6G. A photochemical laser ejection model is proposed to-account for the energetic nonthermal velocity di stributions measured.