Pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy of metastable He2: Ionization potential and rovibrational structure of He2+

Abstract

A supersonic beam of metastable He(*) atoms and He(2) (*) a (3)Sigma(u) (+) molecules has been generated using a pulsed discharge at the exit of a pulsed valve prior to the gas expansion into vacuum. Pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the He(2) (+) X(+) (2)Sigma(u) (+) (v(+)=0-2)<--He(2) (*) a (3)Sigma(u) (+) (v(")=0-2) transitions and photoionization spectra of He(2) (*) in the vicinity of the lowest ionization thresholds have been recorded. The energy level structures of (4)He(2) (+) X(+) (2)Sigma(u) (+) (v(+)< or =2,N(+)< or =23) and (3)He(2) (+) X(+) (2)Sigma(u) (+) (v(+)=0,N(+)< or =11) have been determined, and an accurate set of molecular constants for all isotopomers of He(2) (+) has been derived in a global analysis of all spectroscopic data reported to date on the low vibrational levels of He(2) (+). The analysis of the photoionization spectrum by multichannel quantum defect theory has provided a set of parameters describing the threshold photoionization dynamics.

Transport coefficients of He+ ions in helium

“…The level of agreement between the RCCSD(T) and CAS+MRCI potentials is remarkably good throughout. In addition, the agreement with the spectroscopic constants derived from the PFI-ZEKE spectra of Raunhardt et al 48 is almost always within the experimental error bars, again attesting to the high quality of the present potential, obtained using "standard" methods; we recall that we obtained exceptional agreement with the 1-3 and 1-5 N spacings for more recent PFI-ZEKE results from the same group. The fact that we are making use of the X -3 extrapolation of potentials makes this agreement all the more encouraging, particularly since the (5,6) potential appears to be extremely close to that obtained from the more prohibitive (6,7) method; even the (Q,5) potential seems to be "good", but is not at the high level of the other two potentials; additionally, including a second set of diffuse functions makes little difference to the potentials.…”

Slow and velocity-tunable beams of metastableHe2by multistage Zeeman deceleration

“…As explained in Ref. [38], removal of the 3sσ electron leads to a dominant Q-type rotational branch in the PFI-ZEKE photoelectron spectrum of He * The higher signal-to-noise ratio obtained at a nozzleassembly temperature of 10 K indicates that the density of He * 2 is almost an order of magnitude larger than when the nozzle assembly is operated at room temperature. Whereas the rotational-line-intensity distribution of the spectrum recorded following expansion from the 300 K nozzle is well described by a rotational temperature of about 150 K, the spectrum of the He * 2 sample obtained with the 10 K nozzle assembly is characterized by a bimodal rotational intensity distribution, with a cold component corresponding to a temperature of about 150 K for levels with N ≤ 7 and a much hotter (nonthermal) component corresponding to rotational levels in the range N = 9−21 with a maximum at N = 17.…”

“…Such conditions typically prevail in plasmas, such as those generated by striking an electric discharge through helium gas [37,38]. The large ratio of magnetic moment to mass makes He * 2 particularly attractive for multistage Zeeman deceleration.…”

“…photoionizing transition [38]. The internal state distribution of the metastable molecules was deduced from the rotational structure of the photoelectron spectra and the analysis of the Rydberg series observed in the photoionization spectra, as explained in Section III A.…”

High‐resolution Photoelectron Spectroscopy

“…Microwave, infrared and electronic spectroscopy have been exceptionally successful in providing spectroscopic information on fundamentally important cations such as H + 3 (Oka and Jagod 1993), CH + 2 (Rösslein et al 1992), CH + 3 (Crofton et al 1985), CH + 5 (White et al 1999) and H 3 O + (Begemann et al 1983), to list but a few. Other important cations, such as H + 2 , He + 2 , or CH + 4 have been difficult to study with these methods, and, so far, could only be investigated at highresolution by photoelectron spectroscopy and related methods (Osterwalder et al 2004b, Raunhardt et al 2008, Signorell and Merkt 2000, Wörner and Merkt 2009. It thus appears that direct spectroscopic methods and methods related to photoelectron spectroscopy are complementary, not only as far as the molecular cations that can be investigated, but also as far the states that are accessible by virtue of the different selection rules are concerned: Using photoelectron spectroscopy, information on the cations is obtained by photionizing neutral molecules, which facilitates certain aspects of sample preparation and handling.…”