The 69 th International Symposium on Molecular Spectroscopy, June 2014 U. Illinois Champagne-Urbanna, Timothy C. Steimle, Hailing Wang a and Ruohan Zhang Dept. Chem. & BioChem., Arizona State University, Tempe, AZ,USA Funded by: NSF Optical Stark Spectroscopy of MgD* A Visiting from: State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, , China (0,0)A 2  -X 2  + The deuteride isotopologue is studied instead of the hydride because the Stark shift in the X 2 Σ + (v=0) state in the former is significantly larger than that of the latter. *

Goal: Determine  el and improved spectroscopic parameters. a)Slowing and trapping: MgH is light and has both an electric dipole and magnetic dipole. Mg rod Alternating Grid Stark decelerator Yin Jian-Ping & Hailing Wang- Shanghai b) Assessing the various computational predictions, of which there are many: “Ab initio potential energy curves and transition dipole moments for the X 2 Σ +, A 2 Π and B' 2 Σ+ states of MgH”, M. Mostafanejad and A. Shayesteh, Chem.Phys. Letts. (2012), 551, “ Accurate potential energy functions and non-adiabatic couplings in the Mg + H system” M. Guitou, A. Spielfiedel, and N. Feautrie, Chem.Phys. Letts. 488 (2010) 145–152. “Ab-initio calculation of the ground and excited states of MgH using a pseudopotential approach” J.-M. Mestdagh, P. de Pujo, B. Soep and F. Spiegelman, Chem. Phys. Letts. 471 (2009) 22–28. “ Hyperfine coupling constants, electron-spin g-factors and vertical spectra of the X 2  + radicals BeH, MgH, CaH and BZ +, AlZ +, GaZ + (Z = H, Li, Na, K). A theoretical study.” P.J. Bruna and F. Grein, Phys. Chem. Chem. Phys., 2003, 5, 3140–3153. Motivation:

b) Assessing the various computational predictions of which there are many (cont.): “Relativistic calculation of hyperfine and electron spin resonance parameters in diatomic molecules” H.M. Quiney, and P. Belanzoni, Chem. Phys. Letts. 353 (2002) 253–258 “Ab Initio configuration interaction study of the low-lying electronic states of MgH ” R.P. Saxon, K. Kirby and B. Liu, J. Chem. Phys. 12, (1978), “Roaming dynamics in the MgH + H  Mg + H 2 reaction: Quantum dynamics calculations” T. Takayanagi and T. Tanaka Chem. Phys. Letts 504 (2011) 130–135 “Quantum Manifestation of Roaming in H + MgH → Mg + H 2 : The Birth of Roaming Resonances” A. Li, J. Li, and H. Guo J. Phys. Chem. A 2013, 117, 5052−5060. MgH is a 13-electron problem with minimum relativistic effects and electron correlation. Electronic structure calculations should very accurately predict ground state PES and dipole moments. Low-lying excited states could be problematic (next slide):

Dipole moment as a function of internuclear Separation Pot. Energy curves B ’ 2  + A2A2 X 2  +

Previous Experimental Studies: Too numerous to list all: see Bernath’s group’s publications Those most relevant to current study of MgD: 1. “Fourier transform infrared emission spectra of MgH and MgD” Shayesteh, A., Appadoo, D. R. T., Gordon, R., Le Roy, R. J. & Bernath, P. F. 2004, J. Chem. Phys, 120, X 2 Σ + (v=0) field- free parameters of MgD are very well determined. Combined fit of FTIR, Diode laser (Pines’ data), LMR (Evenson’s data), mm-wave data (Ziury’s data) 2. “Accurate Analytic Potential and Born−Oppenheimer Breakdown Functions for MgH and MgD from a Direct-Potential-Fit Data Analysis” R. D. E. Henderson, A. Shayesteh, J. Tao, C. C. Haugen, P. F. Bernath, and Robert J. Le Roy J. Phys. Chem. A 2013, 117, 13373− Improved analytical potential energy and Born-Oppenheimer breakdown functions for the X 2  + state were derived but no spectroscopic parameters for the A 2  state were given. Most of the low-J branch features of the (0,0)A 2  -X 2  + band have not been previous detected or analyzed.

Well collimated molecular beam Rot.Temp.<10 K Single freq. tunable laser radiation PMT Gated photon counter Experimental set up for LIF/Stark studies Stark plates Optical Stark spectroscopy Metal target Pulse valve skimmer Ablation laser D 2 & Carrier Mg tube

Laser Ablation and Molecular Beam Production 0.30’’ Mg rod Ablation laser Molecular beam Pulsed valve

Observations –Field-free spectrum Abundances: 26 Mg (11%); 25 Mg (10%); 24 Mg (79%); 25 Mg (I=5/2) hyperfine spit X 2  v=0  N=1 J=1/2 J=3/2 A 2  (v=0) J=3/2

Observations –Stark spectrum of R 1 (1/2) line LevelsSpectra BACD

Analysis : Field free B,D, , Fixed to values Bernath’s 2004 manuscript T,A,B,p,q,and , optimized in least squares fit of transition wavenumbers. Optimized A 2  (v=0) state parameters (in cm -1 ): 45 precisely measured field-free transition wavenumbers Standard deviation of the fit.

Analysis : Stark X 2   (v=0) J=1/2,3/2, 5/2, and 7/2 8  8 matrix Diagonalization Eigenvalues for X 2   (v=0) A 2  (v=0) J=1/2,3/2, 5/2, and 7/2 16  16 matrix Diagonalization Eigenvalues for A 2  (v=0) Least Sq. fitting. 51 precisely measured Stark shifts  (X 2  + ) = 1.31(8) D  (A 2  ) = 2.567(10) D Standard deviation of fit: 13 MHz

Discussion  MgD  (X 2  + ) = 1.31(8) D  MgH  (X 2  + ) = 1.32(8) D 1.32(8) Exp SCF-SDTCI a R.P. Saxon, K. Kirby and B. Liu, J. Chem. Phys. 12, (1978), SCF-SDTCI a) 1.37 B3LYP G(2df,2pd) b b) Method Value (D) X2+X2+ Method Value (D) 2.63 SCF-SDTCI a 2.567(10) Exp. A2A2 The DFT and ab initio predictions for both X 2  + and A 2  are slightly larger than the experimental values.

Summary The low-J lines of the A 2  state are not perturbed unlike the higher J-values ( see Bernath’s work). An improved set of spectroscopic parameters for the A 2  (v=0) state have been derived. A molecular beam sample has been studied for the first time. The permanent electric dipole moments for the A 2  (v=0) and X 2   (v=0) states have been determined. Thank you !