Masato Hayashi & Yasuhiro Ohshima Department of Photo-molecular Science Institute for Molecular Science (IMS) National Institutes of Natural Sciences & SOKENDAI (Graduate Univ. for Advanced Studies) Okazaki, JAPAN Sub-Doppler Electronic Spectrum of the Benzene–D 2 Complex International Symposium on Molecular Spectroscopy 69 th Meeting, Univ. Illinois at Champaign-Urbana, IL, USA June 16-20, 2014

Introduction: Complexes with H 2 /D 2 Small anisotropy for the rotation of H 2 /D 2 in the complexes Permutation of two H/D nuclei is feasible. oH2oH2 Previous works : clusters with HF, HCl, CO, H 2 O, HCN,… We have recorded high-resolution spectrum of benzene–H 2 to discuss the H 2 internal rotation. JPCA 117, 9819 (2013). The potential barrier was evaluated, but there remained two possibilities due to some ambiguity in vibrational assignment. pH2 pH2

Experimental Setup CW Ring TiS Sirah Matisse TS Pulsed Amplifier Sirah PDA Isolator LBO 532 nm ~250 mJ ~780 nm ~20 mJ CW DPSS SP Millennia X /2 PDA Matisse Pro-190 Meter ~780 nm, ~300 mW BBO Injection-seeded Nd:YAG SP Pro-190 Dye ND 6000 Nd:YAG Surelite ~260 nm < 0.1 mJ ~280 nm MCP R 0 (0) of benzene (Bz) monomer FWHM ~230 MHz benzene 80 Torr w/ D 2 (10-20%) in atm He

The S 1 –S Band of Benzene–D 2 The vicinity of the benzene (Bz) monomer band was searched by 1+1’ REMPI. 38,65038,700 Wavenumber/cm ‒ 1 Low resolution scan w/ Bz + normalD 2 (oD 2 :pD 2 = 2:1) sample Bz + –D 2 mass channel monitor Bz ‒ pD Bz ‒ oD of Bz monomer (38,606 cm ‒ 1 ) H2H2 D2D2 ｊ = even ｊ = odd I = 0 I = 0 & 2 I = 1 para(pH 2 )ortho(oH 2 ) para(pD 2 )ortho(oD 2 ) oD 2 : pD 2 = 2 : 1 pH 2 : oH 2 = 1 : 3

High-Res Spectrum of the S 1 –S Band Calc. Obs. T rot = 0.3 K Calc. Obs. T rot = 0.3 K Bz ‒ pD 2 Bz ‒ oD 2 Both are perpendicular bands. Bz ‒ oD 2 B” /cm ‒ 1 C” /cm ‒ 1 B’ /cm ‒ 1 C’ /cm ‒ 1  eff ’ 0 /cm ‒ (2) a (13) (7) ‒ (14) 38, (5) a Fix to the monomer C” constant. Numbers in parentheses are  of fit. B” /cm ‒ 1 C” /cm ‒ 1 B’ /cm ‒ 1 C’ /cm ‒ 1  eff ’ 0 /cm ‒ (9) a (6) (4) ‒ (9) 38, (4) Bz ‒ pD 2 38,63538,63638,637 38,62838,62938,627 Wavenumber/cm ‒ 1

Vibronic Bands Pertinent to vdW Modes 38,650 38,700 w/ Bz + normalD 2 sample Bz + ‒ D 2 mass channel ×4 Bz ‒ (pD 2 ) 2 Bz ‒ (pD 2 ) vdW vibrations All of them are perpendicular bands. Low resolution scan Bz ‒ pD 2 Bz ‒ oD 2 S 1 –S

vdW Vibration and Selection Rules in Bz–D 2 Intermolecular modes stretch bendtwist Bend : v b, l b = v b, v b ‒ 2, …, ‒ v b Twist : Correlating to internal rotation of D 2 l = l b + m l b : Angular momentum m : Angular momentum g ev = g Bz + l g Bz : Angular momentum associated to benzene (  g Bz =  1) l : Angular momentum associated to the intermolecular modes Perpendicular band g ev : Total vibronic angular momentum  g ev = g ev ’ ‒ g ev ” =  1  l = 0, 2 But, their intensities should be:  l = 0 >  l = 2

Internal Rotation in Benzene–D 2 (H 2 ) Twist(ortho) > 185 cm ‒ 1 Twist(para) > 0 cm ‒ 1 for Benzene ‒ D m =±1 m = 0 m = ±1 oD2 ｊ = 0 pD 2 ｊ = 1 oD2 ｊ =2oD2 ｊ =2 Free Hindered v = 1 v = 0 Energy/cm ‒ 1 V 2 /cm ‒ 1 m = ±2 m = 0 v = 2 b : rotational constant of H 2 (= 62.3 cm ‒ 1 ) or D 2 (= 31.3 cm ‒ 1 ) V(  ) = V 2 P 2 (cos  ) = V 2 (3cos 2  ‒ 1)/2 H = bj 2 + V(  )

Assignment of vdW Bands 38,650 38,700 ×4 Bz ‒ pD 2 Bz ‒ oD 2 S 1 –S Wavenumber/cm ‒ s01601s01 Perpendicular bands →  l =  (l b + m) = 0 Then, pD 2 : s 0 1, b 0 2, or b 0 1 t 0 1 oD 2 : s 0 1 or b 0 2 Intermolecular modes stretch bendtwist 601b02601b b01t01601b01t01

Comparison with Benzene ‒ H 2 38,650 38,700 ×4 Bz ‒ pD 2 Bz ‒ oD 2 Wavenumber/cm ‒ 1 601b02601b b01t01601b01t s01601s01 Bz ‒ (pD 2 ) 2 Bz ‒ (pD 2 ) 3 S 1 –S Bz ‒ oH 2 Wavenumber/cm ‒ 1 38,65038,700 Definitive assignment! 601s01601s01 601b02601b02 601b01t01601b01t01 Bz ‒ (oH 2 ) 2

Vibrational Frequencies for vdW Modes Wavenumber/cm ‒ 1 Bz ‒ pD 2 Bz ‒ oD 2 Bz ‒ oH 2 Bz ‒ pH 2 Bz ‒ H 2 /Bz ‒ D 2 Stretch /cm ‒ /1.11 Bend /cm ‒ 1 (71.2/2=) (77.2/2=) Twist /cm ‒ 1 ( =) 26.7 ( =) Internal rotation Large deviation from harmonic value (1.41) 38,650 38,700 ×4 Bz ‒ pD 2 Bz ‒ oD 2 601b02601b b01t01601b01t s01601s01 Bz ‒ (pD 2 ) 2 Bz ‒ (pD 2 ) 3 S 1 –S

Internal-Rotation Potential b : rotational constant of D 2 (= 31.3 cm ‒ 1 ) V(  ) = V 2 P 2 (cos  ) = V 2 (3cos 2  ‒ 1)/2 H = bj 2 + V (  ) para ortho Free:V 2 = 0 Energy/cm ‒ 1 m = ±1 m = 0 j = 0 j = 1 Bz ‒ D 2 Bz ‒ H 2 Twist /cm ‒ V 2 /cm ‒ 1 ‒ 44 ‒ 48 D 0 [Bz + D 2 (j) → Bz ‒ D 2 (j)]: para(j = 1) > ortho(j = 0) cf. D 0 (Bz ‒ He) ~ 30 cm ‒ 1 D 0 (Bz ‒ Ar) ~ 330 cm ‒ 1 from dynamical calc. w/ CCSD(T) IPSs Twist(para) 26.7 cm ‒ 1 V 2 = ‒ 44 cm ‒ 1

Conclusion Intermolecular vibrational frequencies of benzene ‒ pD 2 /oD 2 have been determined, and the assignment on the vibronic bands of benzene ‒ H 2 has been fixed. Almost full rotational resolution was crucial for the definitive assignments, as well as precise evaluation of the molecular constants. Analysis for the H 2 /D 2 internal rotation has been made to determine the anisotropy for the hindered rotation in the binary clusters. The intermolecular modes are substantially anharmonic, as evidenced by the large deviation in the isotropic ratios of their frequencies from the harmonic values. The present results should be valuable experimental inputs to reconstruct the global potential energy surface for the prototypical system to study the intermolecular interaction with aromatic rings. Acknowledgments Grants-in-Aid from JSPS/MEXT RIKEN/IMS Joint Program “Extreme Photonics” Consortium for Photon Science and Technology

Thank you for your attention & glad to answer your questions The Castle IMS Okazaki City ( ca. 200 years ago; Edo period) By Hiroshige Utagawa (Ando)

About 0.1 Å shorter Difference in the zero-point vibration of H 2 /D 2 Intermolecular Distances R/ÅR/Å j = 1j = 0 Bz ‒ pD 2 Bz ‒ oH 2 Bz ‒ oD 2 Bz ‒ pH 2 S0S (4)3.461(5)3.258(6)3.22(7) S161S (3)3.593(5)3.341(6)3.45(5) S161s1S161s (6)3.78(1)3.54(1)3.8(1) S161b2S161b (6)3.573(3)3.661(9) S161b1t1S161b1t1 3.84(2)3.91(1)

Polarizability/Å 3 R/Å He Ne Ar Kr Xe N2N2 oH2oH2 : S 0 : S R(S 0 ) < R(S 1 ) Destabilization by the electronic excitation Benzene ‒ X complexes Intermolecular Distance vs Polarizability D 0 from dynamical calc. w/ CCSD(T) IPS ~330 cm ‒ 1 ~400 cm ‒ 1 ~30 cm ‒ 1 pD2pD2

Band Shifts vs Polarizability Benzene ‒ X complexes Blue shifts for “protic solvent (X)” Linear correlation of red shifts against polarizability oD2oD2 pD2pD2

Band Shifts vs Cluster Size oD2oD2 pD2pD2

Line width and Excited-State Lifetime  /GHz  /ns oH2oH (2)1.01(11) 61s161s1 0.27(2)0.58(4) 61b1t161b1t1 0.35(6)0.46(8) 61b261b2 0.42(6)0.38(5) (oH2)2(oH2) (1)0.70(4) 61s161s1 0.43(5)0.37(5) (oH2)3(oH2) (2)0.13(2) pH2pH (4)0.38(4) 61s161s1 0.50(5)0.32(3) (pH2)2(pH2) (9)0.22(3) Fitted to Voigt function R(0) Lines Gaussian w/ FWHM ~230 MHz pD2pD (1)0.52(3) 61s161s1 0.46(9)0.34(7) 61b1t161b1t1 0.49(10)0.32(7) 61b261b2 0.45(10)0.35(7) oD2oD (3) 61s161s1 0.41(6)0.39(5) 61b261b2 0.42(9)0.38(8)

Bz ‒ oH 2 Perpendicular bands B” /cm -1 C” /cm -1 B’ /cm -1 C’ /cm -1  eff ’ 0 /cm (3) a (3) (3) (10) (7) B” /cm -1 C” /cm -1 B’ /cm -1 C’ /cm -1  eff ’ 0 /cm (4) a 0.145(3) (12) -0.58(2) (4) Bz ‒ pH 2 a Fix to the monomer C” constant. Wavenumber/cm -1 Numbers in parentheses are 3  of fit. High-Res Spectrum of the S 1 –S Band Bz ‒ oH 2 Bz ‒ pH 2

Bz ‒ oH 2 Bz ‒ pH 2 Bz ‒ (oH 2 ) 2 Bz ‒ (pH 2 ) vdW vibration Wavenumber/cm  Bz + ‒ H 2 mass channel   Bz + pH 2 ( >99.9%) Bz + normalH 2 (oH 2 :pH 2 = 3:1) Vibronic Bands Pertinent to vdW Modes All of them are perpendicular bands

Benzene–(oH 2 ) 3 Obs. w/ 10% nlH 2 in He ~50 bar m d = 0 Scrambling? m d =  1 Wavenumber/cm -1