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On Metal‐Atom Clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali‐metal molecules
Author(s) -
Herrmann André,
Leutwyler Samuel,
Schumacher Ernst,
Wöste Ludger
Publication year - 1978
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19780610141
Subject(s) - photoionization , chemistry , ionization , atomic physics , excited state , spectral line , atom (system on chip) , ion , physics , organic chemistry , astronomy , computer science , embedded system
The investigation of electronic and structural parameters of metal molecules as a function of size may be decisive for understanding and control of heterogeneous catalysis with finely divided metals. Metal‐atom clusters can be prepared by several methods, most of which yield a molecular mixture only. Expansion of an atomic vapour into vacuum through a supersonic nozzle creates a complex cluster‐spectrum which has been investigated by photoionization using a mass‐spectrometer as selective detector. Broad band single photoionization ( PI. ) yields the abundances and ionization thresholds for Na x (x⩽16), K x (x⩽12) and Na x K y (x+y⩽6). Rough indications about other deactivation channels of excited alkali‐molecules are obtained from an analysis of the photoionization efficiency (PIE.) curves. Two‐photon PIE. curves with narrow‐band (laser) light sources give an accurate photoionization threshold value and detailed information on ionization processes. Two‐photon ionization spectra via a real intermediate state reached by laser excitation are equivalent to normal absorption spectra, if the exciteation step is controlled by the true transition probabilities. By investigation of the power dependence of the ion current of Na 2 +as a function of the wavelength of the tunable cw dye laser (excitation) and of the ionizing Ar + ‐ or Kr + ‐laser, conditions under which true spectra are obtained have been clarified, in good agreement with a photon‐kinetic model of the processes involved. Vibronic and rovibronic spectra of several transitions in Na 2 and K 2 have been measured. Similarly a spectrum of Na 3 has been determined mass‐selectively. The scope of the new method for an absorption spectroscopy in molecular beams is discussed.