June 22nd 2010 65 OSU Mol. Spect. Symp.TA01 8:30 a.m 1 Time Dependent Measurements of Nitrous Oxide-...

June 22nd 2010 65 OSU Mol. Spect. Symp.TA01 8:30 a.m

1

Time Dependent Measurements of Nitrous Oxide-Foreign Gas Collisional Relaxation Processes Using a

Frequency Down-Chirped 7.84 Microns Quantum Cascade Laser

Kenneth Hay, Geoffrey Duxbury Nigel Langford and David Wilson

Department of Physics University of Strathclyde Glasgow, Scotland

and

Nicola Tasinato, Universita Ca Foscari di Venezia,Venice, Italy


2

Outline of Talk• Intra-pulse spectrometers: rapid passage and

molecular alignment• Chirp induced “rapid passage” effects in the

spectrum of nitrous oxide Signatures of molecular alignment

• Collisional effects on gain and absorption• Chirp rate dependent scattering processes• Analogy with chirped frequency Microwave

spectrometers• Conclusions


3

Doppler limited Experiments: The Intra-Pulse Method

• Apply a 50 - 2000 ns top hat current pulse to a DFB QC laser.

• Obtain a light pulse in

time domain with a frequency down chirp.

• Pass pulse through absorbing species and monitor pulse absorption in time domain.

• Laser and Vigo MCT detector both Peltier cooled, no liquid nitrogen needed


4

Transmission spectra of a section of the band of nitrous oxide

green slow, red fast chirp ratea) Spectrum as a function of time, b) total spectrum as a function of wavenumber.Conversion from time to relative wavenumber via Ge etalon fringes

ν1


5

Rapid Passage and Molecular AlignmentRapidly frequency down-chirped radiation interacts with absorbing gas molecules.

Interaction time determined by chirp rate and linewidth of transition.

|dν/dt| = 40 MHz/ns, ∆νD = 80 MHz (typical Doppler linewidth). Interaction time 2 ns.

Interaction time less than Doppler de-phasing time (~1/∆νD), ~12.5ns, and much less than rotational de-phasing time, ~100 ns at 1 Torr.


6

R branch structure of transition in the 10002-00001 band of 16O12C18O

• Path length 37 m• (74 passes)• Laser T -20 C, Drive

voltage 15• Pulse duration 1500

ns• Rep. rate 2.5 kHz• Cumulative average of

8000 spectra• Total pressures, Torr


7

Propagation in an optically dense medium with rapid chirp rate

Delayed signals occur when a chirped pulse propagates through an optically dense medium with minimal collisional damping.

The inference from our Maxwell -Bloch calculations (see references) is that a nutation results from a rapid driving between the upper and lower molecular vibration-rotation levels at the Rabi frequency.

The transient gain is due to constructive interference between the incident laser field and the field generated by the molecular response.

Constructive interference occurs when the chirp rate of the laser is faster than the molecular collisional reorientation time.


8

Pressure dependence of the rapid passage structure part of the P branch of nitrous oxide

High pressure, very large gain signals


9

Number density equivalence of the rapid passage structure of absorption lines of nitrous oxide

Note similarity of the rapid passage structures, of P9 and the R8 and P4 lines of the isotopologues when their number densities are equivalent, and the variation of the rapid passage structure woth chirp rate.


10

Quenching the induced polarization

of the P(9) line of N2O (a) He

(b) Ar

(c) N2

(d) CO2


11

Broadening of the absorptive part of the P(9) line of N2O

(a) He

(b) Ar

(c) N2

(d) CO2


12

Normalised sweep rate and rapid passage

Buffer gas pressure> 20 Torr, a>>1

Gain due to self induced heterodyne depends on sweep rate

Collision dominated20-100 Torr, a< 10

Normalised sweep rate, a

a=dν dtγ1γ2

≈dν dtγ2

if γ1 =γ1 =γ


13

Linear dependence of the Lorentzian half widths on the gas buffer pressure for the P(9) (upper

panel) and P(10) (lower panel) transitions (, green) helium broadening, (, red) argon broadening, (, black) nitrogen broadening, (, blue) carbon dioxide broadening. The error weighted linear fits used to retrieve the effective broadening parameters are also displayed.


14

Retrieved effective nitrous oxide – foreign gas broadening parameters, γ0, and their

dependence on the chirp rate(, green) , (, red) ,

(, black) , (, blue) . Open, half-open and solid symbols refer to P(9), P(10) and P(11) transitions respectively.


15

Conclusions:Propagation with slow chirp rate in an optically thin or optically thick medium

Rapid chirp QC laser spectroscopy is a powerful tool for studying fundamental processes in non-linear optics and molecular physics

More complete model, need to consider a set of open two level systems to aid the interpretation of the experimental data.

At low pressure, collisions causing velocity and speed changing, and rotational energy transfer, become important

Advantage is the ability to probe time dependent collisional effects, overlap with molecular beam scattering experiments

A modern microwave “broadband Fourier Transform” version of the long chirp excitation of molecular coherence was described by Pate and his group in 2008. Brown et al. Rev Sci Inst 79, 053103 (2008). Many papers using this technique have been presented here recently.


16

Acknowledgements We are indebted to the the ERPSRC for an instrumentation grant and for the awards to K.G.

Hay and D.Wilson of studentships through the Doctoral Training Fund,and to NERC for the award of a COSMAS grant for the development of the intra-pulse spectrometer.

Nicola Tasinato is very grateful to the Universita Ca’ Foscari di Venezia for a studentship, and for support of his collaboration with the Strathclyde University group.

GD has greatly benefited from the award of an Emeritus Followship by the Leverhulme Trust .

We would also like to thank Cascade Technologies for their technical support..


17

References1] G. Duxbury, N. Langford, M.T. McCulloch and S. Wright, “Rapid passage induced

population transfer and coherences in the 8 micron spectrum of nitrous oxide”, Mol. Phys.105, 741 (2007)

2] G. Duxbury N. Langford and K. Hay, “Delayed rapid passage and transient gain signals generated using a chirped 8 micron quantum cascade laser”, J. Mod. Opt. 55, 3293 (2008)

[3] N.Tasinato, N.Langford, G.Duxbury and K.G. Hay, “An investigation of collisional processes in a Dicke narrowed transition of water vapor in the

7.8 micron spectral region by frequency down-chirped quantum cascade laser spectroscopy “J.Chem. Phys. 132, 044316 (2010)

[4] N.Tasinato, K.G. Hay, N.Langford, G.Duxbury and D. Wilson,“Time dependent measurements of nitrous oxide and carbon dioxide collisional relaxation processes by a frequency down-chirped quantum cascade laser: Rapid passage signals and the time dependence of collisional processes”,J.Chem. Phys. 132, 164301 (2010

μ

June 22nd 2010 65 OSU Mol. Spect. Symp.TA01 8:30 a.m 1 Time Dependent Measurements of Nitrous Oxide-...

Documents

Transcript of June 22nd 2010 65 OSU Mol. Spect. Symp.TA01 8:30 a.m 1 Time Dependent Measurements of Nitrous Oxide-...