Rotatable source crossed molecular beams apparatus with pulsed ultravioletvacuum ultraviolet photoionization detection

A newly constructed universal crossed molecular beams apparatus for studies of bimolecular chemical reaction dynamics is described. The apparatus empl oys two rotatable molecular beam sources and a fixed ultrahigh vacuum (UHV) quadrupole mass spectrometer with electron impact or pulsed photoionizatio n of reaction products. Electronically cold neutral supersonic transition m etal atomic beams are produced in one of the rotatable sources using laser vaporization. The beams are characterized by laser induced fluorescence spe ctroscopy, photodepletion spectroscopy, and time-of-flight analysis. Photoi onization of the ZrC2H2 products from the crossed beam reaction Zr+(CH4)-H- 2-->ZrC2H2+H-2 is carried out using the pulsed 157 nm radiation from a F-2 excimer laser in the UHV region of the mass spectrometer. Compared to conve ntional electron impact ionization, 157 nm photoionization improves signal- to-noise ratios by more than two orders of magnitude for experiments using pulsed beam sources where cross correlation methods cannot be used. We also demonstrate the use of 1 + 1 resonance enhanced multiphoton ionization for state selective detection of nonreactively scattered transition metal atom s from decay of long-lived collision complexes. Due to the small reaction c ross sections for Y+C2H6-->YC2H4+H-2 and Y+CD3CDO-->DYCD3+CO, these reactio ns could not be studied using electron impact ionization. However, photoion ization detection permitted direct studies of the reactions with excellent signal-to-noise ratios. The greatly improved sensitivity of the photoioniza tion technique facilitates studies of transition metal systems not previous ly amenable to the crossed beams method. (C) 1999 American Institute of Phy sics. [S0034-6748(99)03806-X].