1. The Quintet of HF Dimer (I) The (4,0) mode at 14700 cm -1 Zhenhong Yu, Essam Hammam, and William Klemperer Department of Chemistry and Chemical Biology Harvard University

2. Hydrogen interchange tunneling of HF dimer FFFF HHHH

3. Phenomenological model for hydrogen interchange tunneling Energy level of local oscillator:  = -0.33 cm -1 = -11 cm -1

4. Hydrogen interchange tunneling of (HF) 2 at (v,0)  t (K=0)  t (K=1) Obs. Calc. (0,0) (1,0) (2,0) (3,0) (4,0) 19.747(19.75)31.911(31.91) 6.4626.9110.48910.55 6.3520.362.8240.60 0.072<0.10<0.16<0.10 <0.03

5. The predicted vibrational frequency and tunneling splitting of (HF) 2 at N = v 1 + v 2 = 4 ( 1, 2 ) 0 (cm -1 )  t (K=0)(GHz)  t (K=1)(GHz) (4,0) (3,1) (2,2) (1,3) (0,4) 14703.5 15141.1 15240.9 14971.8 14359.8 <0.03 0.25 19.4 0.70 <0.03 0.35 27.2 0.98 <0.03

6. Experimental Scheme HF-HF (40000) Vibrational Predissociation V-V Channel V-R Channel Total  v HF = 2 emission is detected HF(v = 2)+HF(v = 1) HF(v = 3)+HF(v = 0) Excited by tunable dye laser

7. K = 0  0 parallel band of the (4,0)  (0,0) transition of (HF) 2

8. The tunneling doublet of  K = 0 subband The vibrational predissociation linewidth  pd = 470(30) MHz

9. K = 1  0 perpendicular subband of (HF) 2

10. Spectroscopic constants of (HF) 2 at (4,0) (400A + ) (400B + ) (401A + ) (401B + ) 0 (B+C)/2 (B-C) 14700.462(5)0.22270(20) 14700.453(5) 0.22285(24) 14723.608(1) f ; 14723.595(3) e 0.22302(6) 4.4(1)  10 -3 14723.601(2) f ; 14723.585(2) e 0.22289(22) 4.6(2)  10 -3

11. The rotational constants (B+C)/2 at v 1 = 0-4

12. Summary The (4,0) mode of HF dimer has been observed with laser-induced fluorescence. The hydrogen interchange tunneling is basically quenched by the free-HF overtone excitation. The predissociation linewidths of both K = 0 and 1 levels are determined to be 470(30) MHz without apparent rotational dependence.