1. Rotational and vibrational dynamics of water
in superfluid helium droplets hn
Mike Lindsay, Gary Douberly, and
Roger Miller
University of North Carolina at Chapel Hill
OSU International Symposium on Molecular Spectroscopy, TI04, 6/21/2005

2. Narrow inhomogeneous line width
Relaxation dynamics in superfluid helium droplets
Homogeneous & weakly interacting environment
Narrow inhomogeneous line width
(< cm-1)
Homogeneous line widths cm-1 – 5 cm-1
Relaxation time scales 1x10-9 sec – 1x10-12 sec
~ Rotational relaxation ( 1 psec nsec)
~ Vibrational relaxation (1 psec - >2 msec)

3. Why water? Widely spaced vibrational states
Rotational levels form resonances with the
elementary modes of the fluid
Ortho/para metastability
Previous work suggested anomalies *
*Fröchtenicht, Kaloudis, Koch, and Huisken, JCP (1996)

4. Experimental h n Absorption Region: Multipass cell cw-OPO or
F-center laser
Droplet source:
5 µm diameter pinhole
50 Bars He
23 K
<n> ~ 2200 He atoms h n
Pick-up cell:
4 cm length
~10-6 Torr
Detector:
Bolometer
Mass Spectrometer

5. 2.7 µm Spectrum
Fröchtenicht, Kaloudis, Koch, and Huisken, JCP (1996)

6. 2.7 µm Spectrum HDO: Dv0 = +0.35 cm-1 (A’-B’) = -2.7 % ~
(B’+C’)/2 = -3.6 %
H2O:
Dv0 = cm-1
(B+C)/2 = -2.1 %
Field induced Q(0) *

7. Line width comparisons
H2O
t =2.8 (1) ps
t =21 (2) ps
t =18 (2) ps
t =4.0 (3) ps
t =97 (7) ps
t > 800 ps
HDO

8. Line width comparisons
HDO
H2O
t =2.8 (1) ps
t =21 (2) ps
t =18 (2) ps
t =4.0 (3) ps
t =97 (7) ps
t > 800 ps

9. Vibrational Relaxation
Line width comparisons
HDO
H2O
t =2.8 (1) ps
t =21 (2) ps
t =18 (2) ps
t =4.0 (3) ps
t =97 (7) ps
t > 800 ps
Vibrational Relaxation
Only
t H2O, v3=1~ 20 ps
t HDO, v3=1> 800 ps

10. Rotational lifetimes
Hartmann, Mielke, Toennies, Vilesov, and Benedek PRL 76, 4560 (1996)

11. Line width comparisons
HDO
H2O
t =2.8 (1) ps
t =21 (2) ps
t =18 (2) ps
t =4.0 (3) ps
t =97 (7) ps
t > 800 ps
t H2O, v3=1~ 20 ps
t HDO, v3=1> 800 ps

12. Vibrational States
100
001
020
010 ?

13. Does water completely relax to its ground state?h n
Lindsay, Lewis, and Miller, JCP 121, 6095 (2004)

14. Vibrational States Lindsay, Lewis, and Miller, JCP 121, 6095 (2004)
von Haefton and Havenith, (private communication)

15. Summary H2O and HDO dynamics in helium are different
HDO has greater reduction in (B+C)/2
HDO band origin blue shifted; H2O origin red shifted
v3=1 relaxes more quickly in H2O than in HDO
Short lifetime for excited rotational levels
Water relaxes to its ground state on a time scale < 100 µs
Acknowledgements: Klaus von Haeften and Martina Havenith (CO/NO)
Miller Group