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

Narrow inhomogeneous line width Relaxation dynamics in superfluid helium droplets Homogeneous & weakly interacting environment Narrow inhomogeneous line width (< 0.005 cm-1) Homogeneous line widths 0.005 cm-1 – 5 cm-1 Relaxation time scales 1x10-9 sec – 1x10-12 sec ~ Rotational relaxation ( 1 psec - 100 nsec) ~ Vibrational relaxation (1 psec - >2 msec)

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 105 6128 (1996)

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

2.7 µm Spectrum Fröchtenicht, Kaloudis, Koch, and Huisken, JCP 105 6128 (1996)

2.7 µm Spectrum HDO: Dv0 = +0.35 cm-1 (A’-B’) = -2.7 % ~ (B’+C’)/2 = -3.6 % H2O: Dv0 = -0.74 cm-1 (B+C)/2 = -2.1 % Field induced Q(0) *

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

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 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

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

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

Vibrational States 100 001 020 010 ?

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

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

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