2-PHOTON IONIZATION SPECTROSCOPY AND ALL-ELECTRON AB-INITIO STUDY OF LICA

Resonant two-photon ionization spectra of LiCa have been obtained from a laser vaporization, supersonic expansion source. The ground state o f the molecule is confirmed to be X (2)Sigma(+). Three band systems ha ve been observed near 15 282, 19 310, and 22 250 cm-(1) and the upper states have been assigned as (2)Sigma(+), (2)Pi(r), and, tentatively, (2)Pi. Bond lengths and vibrational frequencies are reported for (LiCa )-Li-7-Ca-40 for the various states as X (2)Sigma(+) : r(0) = 3.3796(1 1) Angstrom, Delta G(1/2) = 195.2 cm(-1). (2)Sigma(+)f : r(0) = 3.4275 (47) Angstrom, Delta G(1/2) = 283.5 cm(-1); (2)Pi(r) : r(e) = 3.5451(3 6) Angstrom, omega(e) = 144.5 cm(-1) : and (2)Pi : omega(e) = 178.53(5 ) cm(-1). The ionization energy was also measured to be 4.471(1) eV by observing the onset of one-photon ionization. Results of ab initio al l-electron calculations on twelve low-lying states [(2)Sigma(+)(1), (2 )Sigma(+)(2), (2)Sigma(+)(3), (2)Sigma(+)(4), (4)Sigma(+)(1), (4)Sigma (-)(1), (2)Pi(1), (2)Pi(2), (2)Pi(3), (2)Pi(4), (4)Pi(1), and (4)Pi(2) ] are also reported, along with results on the X (1)Sigma(+) ground el ectronic state of LiCa+. The dissociation energies of LiCa(X (2)Sigma( +)) into Li(S-2) + Ca(S-1) and of LiCa+ (X (1)Sigma(+)) into Li+(S-1) + Ca(S-1) are calculated to be 0.24 and 1.20 eV respectively, and the vertical and adiabatic ionization energies of LiCa have been determine d to be almost the same, 4.43 eV, at the quadratic configuration inter action, including singles and doubles with approximate triples and all correlated electrons [QCISD(T,FULL)] level of theory. (C) 1998 Americ an Institute of Physics. [S0021-9606(98)00240-2].