Note Two-photon excitation of the 2 P(4p)–X 2 P(3p) transition of AlAr Kyle J. Mascaritolo, Ivan O. Antonov, Michael C. Heaven ⇑ Department of Chemistry, Emory University, Atlanta, GA 30322, United States article info Article history: Received 5 November 2013 In revised form 1 January 2014 Available online 13 January 2014 Keywords: van der Waals complex Electronic excitation Potential energy curve abstract The 2 P(4p)–X 2 P(3p) band system of AlAr has been observed using two-photon excitation. The spectrum consists of a short progression of doublet bands, with spin–orbit intervals that are close to that of Al(4p). Potential energy curve fitting yielded a bond dissociation energy for 2 P(4p) of D e = 495(5) cm 1 and an approximate bond length of R e = 3.33(4) Å. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction The Al–Rg complexes (Rg = Ar, Kr, Xe) have been examined pre- viously using electronic spectroscopy techniques [1–4]. These open-shell species are of interest as they can be used to explore the evolution of physical bonding forces as a function of electronic excitation. It has been shown that binding of the lower energy states benefits from the greater polarizability of the excited orbital [3]. At higher energies, where the states become increasingly more Rydberg in character, the Al–Rg binding energy increases towards the value expected for the ionic Al + –Rg complex. To date, spectro- scopic studies of Al–Rg complexes have focused on states that can be accessed by one-photon transitions. In the present study we re- port a band system of Al–Ar, accessed by a two-photon excitation process. Spectra for the B 2 R + (4s)–X 2 P3p transition of Al–Ar were first re- ported by Gardner and Lester [2], who used resonant two-photon ionization (R2PI) as the means for detection of single-photon resonances. This transition was subsequently examined using laser induced fluorescence (LIF) spectroscopy. Callender et al. [1] charac- terized the dispersed fluorescence spectrum, from which they de- rived a ground state dissociation energy of D 0 = 136 ± 65 cm 1 . McQuaid et al. [4] recorded rotationally resolved spectra, from which the B 2 R + (4s) state potential energy curve was recovered via the RKR inversion process. Heidecke et al. [3] used R2PI to study a wide range of states, spanning the range from B 2 R + (4s) all the way to the ionization limit. Term energies and vibrational constants were reported for 33 electronically excited states. The ionization energy was found to be 47418.5(±2.0) cm 1 . In the energy range of relevance to the present study, 32 400–33 000 cm 1 , they observed states that were attributed to the Al(3d) + Ar and Al(4p) + Ar dissoci- ation asymptotes. The two-photon excitation spectra reported here provide new information concerning the 2 P(4p) state. 2. Experimental The apparatus used for these measurements has been described previously [5]. Al–Ar complexes were generated using a conven- tional pulsed laser ablation source [6]. The fundamental from a Nd/YAG laser (1064 nm, 5 mJ/pulse) was used to ablate the sur- face of an Al rod. The ablation plume was entrained in pure Ar at a pressure of 7.6 atm, and cooled by supersonic free-jet expansion. Al–Ar complexes formed during this expansion process. The core of the expansion was sampled via a conical skimmer, into the ion source region of a time-of-flight mass spectrometer. The Al–Ar complexes were photo-ionized by the beam from a tunable dye la- ser, operating over the wavelength range 606.06–617.28 nm. Three photons are required for ionization at this wavelength. As there are no one-photon resonances in this range, resonantly enhanced ion- ization could be unambiguously assigned to a 2+1 sequence. Spec- tra were recorded by monitoring the ion current associated with the arrival time for AlAr + (m/e = 67). The dye laser was operated with a beam diameter of 2 mm and a pulse energy of approxi- mately 5 mJ. The linewidth (FWHM) was 0.3 cm 1 . Absolute wave- length calibration was obtained using a commercial wavemeter (Bristol Instruments model 821). 3. Results and discussion Fig. 1 shows the 2+1 photoionization spectrum. The bands in the low energy range (two-photon energies of 32 300–32 580 cm 1 ) were weak and the vertical scale has been expanded to show them more clearly. The intensities in this trace have not been adjusted 0022-2852/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jms.2014.01.004 ⇑ Corresponding author. E-mail address: heaven@euch4e.chem.emory.edu (M.C. Heaven). Journal of Molecular Spectroscopy 297 (2014) 1–3 Contents lists available at ScienceDirect Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms