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