The Permanent Electric Dipole Moments and Magnetic g e -Factors of Praseodymium Monoxide (PrO) † Hailing Wang, ‡ Colan Linton, § Tongmei Ma, | and Timothy C. Steimle* ,‡ Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604, Centre for Laser Atomic and Molecular Sciences and Physics Department, UniVersity of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick, Canada E3B 5A3, Department of Chemistry, South China UniVersity of Technology, Guangzhou, China ReceiVed: January 22, 2009; ReVised Manuscript ReceiVed: March 10, 2009 The R(4.5) and P(6.5) branch features of the XX (0, 0) band of praseodymium monoxide (PrO) have been studied at a resolution of approximately 50 MHz field free and in the presence of static electric and magnetic fields. The permanent electric dipole moments, µ el , of 3.01(6) D and 4.72(5) D for the X 2 (Ω ) 4.5) and [18.1] (Ω ) 5.5) states, respectively, were determined from the analysis of the Stark spectra. The magnetic g e -factors of 4.48(8) and 5.73(6) for the X 2 (Ω ) 4.5) and [18.1] (Ω ) 5.5) states, respectively, were determined from the analysis of the Zeeman spectra. The g e -factors are compared with those computed using wave functions predicted from ligand field theory and ab initio calculations. The µ el value for the X 2 (Ω ) 4.5) state is compared to ab initio and density functional predicted values and with the experimental values of other lanthanide monoxides. Introduction The very complex optical spectra of the lanthanide monoxides are caused by the insensitivity of the electronic energies to the numerous possible arrangements of the Ln 2+ electrons in the 4f and 6s orbitals. Fortunately, the insensitivity of the electronic energies to the various occupations of the Ln 2+ 4f and 6s orbitals also implies that disentangling the complex optical spectra may be aided by using simple ligand field theory (LFT) to establish the global electronic structure for the low-lying electronic states. Assessment of LFT, as well as more sophisticated electronic structure methodologies, is best achieved by comparing pre- dicted magnetic hyperfine parameters, permanent electric dipole moments, µ el , and magnetic dipole moment, µ m , with experi- mentally measured values because all three properties are sensitive to the various arrangements of the Ln 2+ 4f and 6s electrons. The limited number of valence electrons and the presence of a single isotopologue with nonzero nuclear spin makes praseodymium monoxide, 141 Pr(I ) 5/2)O, a favorable case among the lanthanide monoxide molecules for testing the predictability of LFT and other methodologies. Indeed, the interpretation of the magnetic hyperfine structure in the Doppler- limited laser induced fluorescence (LIF) spectra of PrO, which has been extensively studied by the Field group, 1-4 was instrumental in the original development of LFT for the lanthanide monoxides. 4-8 The magnetic hyperfine structure of the X 2 (Ω ) 4.5) (E ) 220 cm -1 ) and X 1 (Ω ) 3.5) (E ) 0 cm -1 ) states was also precisely characterized from the analysis of the laser-rf double resonance spectrum and interpreted using LFT. 9 Here, we report on the experimental determination of µ el and µ m for the X 2 (Ω ) 4.5) and [18.1] (Ω ) 5.5) states from the analysis of optical Stark and Zeeman spectra for the R(4.5) and P(6.5) lines of the XX (0, 0) band system. The XX (0, 0) band system is the transition between the (V) 0) [18.1] (Ω ) 5.5) and (V) 0) X 2 (Ω ) 4.5) vibronic levels. The results will be compared with values predicted from both LFT and ab initio electronic structure calculations. The small hyperfine splitting in the X 1 (Ω′′ ) 3.5) (E ) 0 cm -1 ) precluded an analysis of Stark and Zeeman effect in the [16.6] (Ω′ ) 3.5)-X 1 (Ω′′ ) 3.5) (E ) 0 cm -1 ) (XVII (0, 0)) band, which was the initial objective of the present study. The magnetic dipole moment of a nonrotating molecule results from a combination of the electronic orbital and electronic spin magnetic dipole moments of the individual electrons. The quantum mechanical operator for the individual electronic orbital and spin magnetic dipole moments are proportional to the individual orbital and spin angular momen- tum operators, l ˆ and s ˆ, respectively. The proportionality constants are simply the Bohr magneton times either the electronic orbital g-factor (g l )() 1) or the spin g-factor (g s )() 2.0023). Thus, µ m can be predicted a priori, given the molecular configurations of a particular electronic state, and conversely, any proposed molecular configuration for a given electronic state must be consistent with an experimentally measured µ m . Unlike µ m , µ el cannot be predicted a priori given a molecular configuration. The permanent electric dipole moment is par- ticularly sensitive to the nature of the chemically relevant valence electrons and is used in the description of numerous phenomena. Accordingly, µ el is routinely predicted from elec- tronic structure calculations either as the expectation value of the dipole moment operator or from analysis of the finite electric field dependence of the energies. The comparison between the expectation value and finite field value is a primary diagnostic of the treatment of configuration interaction. Some time ago, a ground state µ el value of 3.86 D was predicted by a self- consistent field/configuration interactions ab initio calculation using a pseudopotential for the 4f orbitals. 10 A recent density functional theory calculation implementing the B3LYP hybrid functionals predicts a ground state µ el of 4.114 D. 11 The † Part of the “Robert W. Field Festschrift”. * To whom correspondence should be addressed. Phone: (480) 965-3265. E-mail: Tsteimle@ASU.edu. ‡ Arizona State University. § University of New Brunswick. | South China University of Technology. J. Phys. Chem. A 2009, 113, 13372–13378 13372 10.1021/jp900677g CCC: $40.75  2009 American Chemical Society Published on Web 04/20/2009