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Saturation of the NO2 ν1+ν3 and the CH4 ν3 Transitions in Helium
Nanodroplets
Robert Fehnel Kevin Lehmann
Department of ChemistryUniversity of Virginia
Why study Saturation of CH4 and NO2?
• By studying the saturation of these molecules we will try to understand the line shapes in nanodroplets which are inhomogenous.
• By studying relaxation we can try and find the inhomogenous relaxation rates
• Try and understand the relationship between the molecules and a superfluid
Nozzle Diameter = 10 μmSkimmer = 400 μmNozzle T ≥ 16 KBacking Pressure ≤ 60 Bar
LHe
Skimmer
Nozzle
Closed CircuitRefrigeratorsHe
Chopper
Pickup cell
Multipass Cell
1.5K bolometer N.E.P. ~ 2x10-14 W/Hz1/2
10 20 30 45 cm>5000 L/s 2500 L/s
IR OPO 2560 – 3125 cm-1
Machine Schematic
Bolometer noise ~ beam noise ~ 10-5 of chopped beam signal(1 Hz BW)
Acculight Argos OPO
S P I
Wavemeter
To Spectrometer
OPO
Powermeter
150 MHz etalon7.5 GHz
etalon
Approximately 1.75 W of power measured entering the polarizer and upwards of 0.7W entering the spectrometer.
Produces over 2 W of CW over the tunable range of 3.2 – 3.9 μm. Continuous scans of 45 GHz. Also produces 2 - 5 W of 1.5 μm light.
MgF2 Polarizer
Perry cell
Power MeterThe Focal Spot was determined to be 27 µm in diameter. Peak power is equal to 240 kW/cm2
He Beam
Lens
Perry Cell Measurements I
Perry Cell Measurements 2
Beam Quality Singe Pass
R2 = 0.9898
Beam Quality Multi Pass
R2 = 0.98015
NO2 spectrum in He~5000
NO2 v1 + v3 R(0) ->
.
NO2 Signal vs PowerThe R(0) Line is found at 2905.566 cm-1 and the FWHM is 0.035 cm-1.
NO2 Signal vs Power
NO2 Signal vs Power
S = a*P/(1+P/Ps)a = 58.954Ps = 0.527(Χ2/(Np-2))½ = 0.451
NO2 Signal vs Power
S = a*P/(1+P/Ps)a = 58.954Ps = 0.527(Χ2/(Np-2))½ = 0.451
S = a*I/((1+P/Ps)½) a = 68.751Ps = 0.117(Χ2/(Np-2))½ = 0.651
NO2 Widths
NO2 WidthsΔν = Δν0((1+I/Is)½)
Homogenous CaseIs = 150 kW/cm2
Δν0 = 0.033(Χ2/(Np-2))½ = 1.9 x10-3
Methane R(0) LineThe R(0) Line is found at 3029.07 cm-1 and the FWHM is 0.20 cm-1.
Methane Signal vs Power
Methane Signal vs Power
S = a*P/(1+P/Ps)a = 41.713Ps = 0.458(Χ2/(Np-2))½ = 0.152
Methane Signal vs Power
S = a*P/(1+P/Ps)a = 41.713Ps = 0.458(Χ2/(Np-2))½ = 0.152
S = a*P/((1+P/Ps)½) a = 49.723Ps = 0.1(Χ2/(Np-2))½ = 0.184
Methane Widths
Methane WidthsΔν = Δν0((1+I/Is)½)
Homogenous CasePs = 0.458Δν0 = 0.172(Χ2/(Np-2))½ = 1.7 x10-3
ResultsNO2 CH4
Is (kW/cm2) 150 130
Transition Dipole (D)
0.05 0.057
T1T2 (ns2) 0.31 0.26T1 (ns) 1.0 5.0
T1T2 = (hbar*ε*c)/(2*μ*Is)
Results II
• Knowing that the focal point diameter is 27 µm and the speed of the beam is 450 m/s then we can determine that the NO2 spends 60ns in each crossing with the beam
• We believe that by comparing our T1 time for methane of 5 ns to a previous result by Momose’ group for the v4 R(0) line of methane which results in a 3.7 ns T2 time that we are relaxing to the 2v4 and then to the ground state
Conclusions
• We were able to show saturation with both CH4 and NO2
• With both species a homogenous and inhomogenous fit worked well for signal
• Only homogenous line shape fit the widths correctly
Future Work
• This technique could be applied to other similar molecules with similar strong lines such as CH3Cl and Propyne
• Also try to adjust the number of passes while keeping the amount of scattered light low– This could be done by putting the Perry cell on a
rotation stage– Also determine saturation by number of passes
instead of using polarizer to adjust power
Acknowledgements
• Dr. Ozgur Birer who help construct the HENDI machine at UVa.
Funding:• National Science Foundation, UVa