Spectroscopy and Dynamics of doped helium nanodroplets Kevin K. Lehmann University of Virginia NSF
Properties of 4 He Nanodroplets Helium binding energy of ~5 cm -1 (7 K) – Evaporative cooling to T ~ 0.38 K – ~100% superfluid if bulk Helium • We generate droplets with <N> ~ 1,000 - 20,000 atoms – R ~ 2 - 7 nm – Broad Log-Normal distribution of size – Broad Log-Normal distribution of size • There are no thermally populated phonons – Lowest excitation energy 534 N -1/3 GHz > 3 kT • Surface ‘Ripplons’ form heat bath ν ( L ) = 78 L ( L − 1)( L + 2) N − 1/2 GHz – Lowest mode (L=2) ~ 2.2 GHz (~0.3 kT) for N = 10 4 – Amplitude ~ 0.4 Å
Why Spectroscopy in He Nanodroplets? • Rapid Cooling (~nsec) below 1 K – Stabilize unstable species and form new compounds • Vibrations and Rotations are cooled • Pickup only requires vapor pressure of ~10 -4 torr – Allows study of very nonvolatile species • Can form controlled clusters • Can form controlled clusters – Poisson Distribution of sizes • Gives Rotational Resolution -> structural information • Higher Resolution and smaller shifts than other Rare gas matrices – The Ultimate Spectroscopic Matrix!
Droplet Production and Detection Microwave NEP: dopant inside Visible, IR, or UV 10 µm aperture flux: (rotational 35-45 (except alkali) T =16–35 K 10 20 atoms/s transitions) fW/ √ Hz ~10 -4 torr He 100 atm laser las cold cluster dopant photon MW multiple (bolometer) detection expansion formation pick-up absorption photon absorption + + He evaporation He evaporation (10 2 -10 4 He/photon) (0.1 He/photon) • Droplet sizes: 1000-10000 atoms (45-95 Å diameter) • Droplet temperature: 0.4 K (evaporative cooling) • Sensitivity (S/N=1@1Hz): 3 × 10 7 He atoms
The cluster machine cold head bolometer bol sour source MW in MW cavity MW cavity MW cavity MW cavity BUC pick-up cell pump gas in out laser in (optic fiber)
The beginning SF 6 in He droplets Goyal, Schutt and Scoles, PRL 69, 933 (1992) •Narrow lines SF 6 •Small shifts •Complexes formed (SF 6 ) 2 // (SF 6 ) 2 ⊥
Rotational Window Cryotip T 0 =5-30 K resolution in Mirror 4 He droplets (Hartmann, Miller, He Interaction region Lens Ionizer P 0 =5-100 bar Toennies, and Vilesov, Scattering & QMS PRL 75, 1566 (1995) gas SF 6 SF 6 SF 6 freely rotates SF 6 freely rotates T =0.37 K T =0.37 K inside the droplet, but P Branch R Branch the observed B is only 37% of the gas phase 4 3 2 1 0 1 2 3 4 value
OCS in 3 He and 4 He droplets: Free rotation is proof of superfluidity in 4 He droplets 4 He He (a exclusive to 4 boson fluid). Grebenev, Toennies, and Vilesov, ������� 279, 2083 (1998) 3 He He (a fermion In 3 fluid) free rotation does not occur, 4 He He unless enough 4 0 is present to form a shell around the 7 OC OC CS molecule. CS molecule. 25 35 60 100 R0 P1 R1 -0.2 0.0 0.2 0.4
Acculight Argos OPO Wavemeter Approximately To Spectrometer 1.75 W of power measured entering the machine. 150 MHz 150 MHz S P S P I I etalon 7.5 GHz etalon OPO Power meter Produces over 2 W of CW over the tunable range of 3.2 – 3.9 � m. Continuous scans of 45 GHz. Also produces 2 - 5 W of 1.5 µ m light.
Methane ν 3 v 3 Q(1) line 3018 3018.5 3019 3019.5 3020 3020.5 wavenumber (cm -1 ) Found two extra lines. A Found two extra lines. A second line near Q(1) and also a P(2) line. P(2) Miller Groups Methane spectra of the ν 3 band. 3001.2 3001.6 3002 3002.4 3002.8 Nauta, K., Miller, R.E. Chem Phys. Let. 350 (2001) 225.
Methane ν 2 + ν 4 2830.8 2831.2 2831.6 2832 2832.4 2845.6 2845.6 2846 2846 2846.4 2846.4 2846.8 2846.8 Found 2 lines of a second band of methane. From these values assuming B’’ and ζ we derived ν 0 = 2836.36 cm -1 and B’ = 5.2279 cm -1 . The addition of an R(1) line would help solve more equations and will be searched for. Line He Gas Phase HWHM Constant He Value Gas Phase Value (cm -1 ) (cm -1 ) (cm -1 ) ν 0 2836.36 cm -1 2841.009 cm -1 Q(1) 2831.459 2835.876 0.051 B’ 4.978 cm -1 5.2279 cm -1 R(0) 2846.304 2851.46 0.068 Lolck, J., Robiette, A., Brown, L., Hunt, R.H. J. Mol. Spec. 92 (1982) 229.
Comparison of Widths Methane v 2 + v 4 CH 2 D 2 v 1 Line He HWHM Line Frequency HWHM (cm -1 ) (cm -1 ) (cm -1 ) (cm -1 ) Q(1) 2831.459 0.051 Q(1) 2977.220 0.0107 R(0) 2846.304 0.068 R(0) 2982.868 0.0184 CH 3 D v 4 CH 2 D 2 v 4 Line Line Frequency Frequency HWHM HWHM Line Line Frequency Frequency HWHM HWHM (cm -1 ) (cm -1 ) (cm -1 ) (cm -1 ) P(1) 3008.846 0.4154 Q(1) 3013.728 0.4421 Q(1) 3016.046 0.221 R(0) 3020.253 0.1556 R(0) 3025.345 0.120 R(1) 3026.077 1.5522 CH 3 D v 1 CHD 3 v 1 Line Frequency HWHM Line Frequency HWHM (cm -1 ) (cm -1 ) (cm -1 ) (cm -1 ) Q(1) 2969.789 0.0289 Q(1) 2993.319 0.0188 R(0) 2977.144 0.0387 R(0) 2999.289 0.0266
CH 3 Cl Line Location HWHM (cm -1 ) (cm -1 ) P 1 (2) 2968.268 0.0423 P 0 (2) 2968.371 0.0334 P 0 (1) 2968.594 2968.594 0.0244 0.0244 Q 0 (1) 2968.78 0.023 R 0 (0) 2969.135 0.0226 R 1 (1) 2969.293 0.032 R 0 (1) 2969.369 0.1041
Planned IR experiments • IR-MW double resonance to determine rate of rotational relaxation in droplets • IR-IR double resonance to determine rate of vibrational relaxation in droplets vibrational relaxation in droplets • Visible-IR double resonance to determine cooling rate of droplets to test evaporative cooling models • Study chemical reactions at very low temperatures – test reactions proposed by Astrochemistry.
New Experiment: Driving Ions in Helium He droplet source Electrostatic energy selector Ions Lens Ion Multipler Atoms h ν grids Figure 3: Schematic of proposed ion machine
(b) (a) Figure 1: Classical Simulation of ion (c) motion in He droplet driving by frequency swept microwave field of 20 V/cm. Red is the frequency; blue the ion kinetic energy; and black to energy deposited in droplet. Not the change in scale on the left for the second figure. This demonstrates that the ion will lock onto and follow the frequency until the ion moves at the critical velocity. Above this, the ion no longer can follow and relaxes to zero energy (c).
Proposed Experiments on ions • Determine critical velocity for different ions & function of droplet size • Measure rate of translational cooling • Use circularly polarized radiation to pump in • Use circularly polarized radiation to pump in angular momentum (~10 9 hbar/sec) – Nucleate vortices? • Measure helium loss vs. energy input – Nanocalorimeter to measure binding energy of clusters formed in droplets.
Evaporative Cooling of Helium Nanodroplets with Angular Momentum Conservation KKL & Adriaan Dokter Physical Review Letters 92, 173401 (2004)
Thermodynamics of Helium Droplets Statistical Evaporative cooling calculations of temperature versus cooling time. D.M. Brink & S. Stringari Z Phys. D 15 257 (1990)
• There have been previous classical and quantum statistical evaporative cooling studies These predicted droplet temperatures close to those later found experimentally They ignored angular momentum constraints • Droplets pick up considerable angular momentum in pickup process and possibly during formation due to droplet coalescence due to droplet coalescence – Can this be completely shed during cooling? – Experiments of Portner, Havenith, and Vilesov interpreted as implying that droplets “remember” direction of initial angular momentum • Droplet Energy and Angular Momentum will be stored in primarily in Ripplons
Distribution of Collisional Angular momentum for N = 10 4 Scales as N 1/3 for other droplet sizes
Monte Carlo Evaporation • Start with E, L from pickup process – E ~ 1700 K and L ~ 4000 for tetracene pickup • Calculate number of open channels – Droplet(E, L) -> Droplet(E’, L’) + He(E k , J) – E k = E - E’ - E bind > 1.23 K J(J+1) N -2/3 (centrifugal barrier) – E k = E - E’ - E bind > 1.23 K J(J+1) N (centrifugal barrier) • Calculate RRKM rate and increase time by lifetime • Pick one decay channel at random equal probability • Repeat until time > cooling time (10 or 100 µ s). • J = 10, 1000, 2000, 3000, 4000 & 5000 studied, 500 decay trajectories for each
Conclusions • Droplets have MUCH higher internal energy and angular momentum than for a canonical ensemble at same temperature. – Explains why previous attempts to predict – Explains why previous attempts to predict lineshapes failed – PIMC will not give exact predictions - may be substantial bias • Molecular Lab frame alignment of embedded molecules should be common.
Energetic Considerations on the formation and decay of a Vortex in Helium Nanodroplets in Helium Nanodroplets KKL & Roman Schmied
• Vortices are common, basically unavoidable in bulk superfluid helium, but have not yet been observed in helium nanodroplets • Simplest Vortex is a straight line – In droplets must go through center – Will have one quantum of angular momentum per – Will have one quantum of angular momentum per helium atom in droplet. – Previous considered by DFT and PIMC calcs. – Helium flows around vortex with local velocity, v = h/2 π mr; r min. distance to vortex
Recommend
More recommend
