Stimulated Infrared Emission of C2H2 near 3000 cm-1

with Continuous-Wave Lasers

Mikael Siltanen,1 Markus Metsälä,1 Markku Vainio,1,2 and Lauri Halonen1

1Department of Chemistry, University of Helsinki, Finland2Centre for Metrology and Accredation, Espoo, Finland

In short• Pump-probe experiment

– based on continuous-wave lasers– molecules are always in the ground electronic state

Pump:

vibrational

overtone

absorption

Probe:

stimulated

emission

• Sample is acetylene, C2H2

– focus on C-H stretching – symmetric states are not accessible with one photon

Ground

Earlier work: Laser induced dispersed fluorescence (LIDF)

• Selective pumping with narrow-line laser

• Spontaneous emission measured with a dispersive instrument (FTIR)

• Provided access to symmetric states

• Inside laser cavity

[M. Metsälä, S. Yang, O. Vaittinen, and L. Halonen, J. Chem. Phys. 117, 8686 (2002)]

Comparison to SEP

• Stimulated emission pumping (SEP)– uses electronic excitation and higher energies

• we use no electronic transitions

– Franck-Condon factors need to be considered• may limit number of accessible states

– normally employs pulsed lasers and background correction

• we need no separate background correction• high resolution achieved with continuous-wave lasers

C2H2 pump beam absorption

[31-] in local mode notation

[40-] in local mode notation

[L. Halonen, Adv. Chem. Phys. 104, 41 (1998)]

Transitions used in the experiments

[40-] (1+3 3)

12 675.68 cm-1

[30+] (1+23)

9 663.36 cm-1

[00+] 0.0 cm-1

State

Probe, ΔJ = ±1

Pump, ΔJ = +1

[31-] (31+ 3)

13 033.30 cm-1

[21+] (31)

9 991.97 cm-1

[00+] 0.0 cm-1

Probe, ΔJ = ±1

State

Pump, ΔJ = -1

Sample cell setup (simplified)

Ring piezoactuator Photo-

diode

LN2-cooledInSb detector

Pressure meter

Gasinlet

Pump beam fromTi:sapphire laser

Probe beamfrom mid-infrared optical

parametric oscillator (OPO)

Dichroicbeam

splitter

Highly reflective mirrorsat probe beam frequency

Oscilloscope

PDH lockfeedback

electronics

1/4-waveplate

Beam-splitter

Acousto-opticmodulator

Photo-diode

Lock-inamplifier

Measuredsignal

C2H2 pressure0.05 – 0.5 Torr

Measurement setup properties

• Pump beam– cw Ti:sapphire near 800 nm / 13 000 cm-1

– chopped at 10-25 kHz– Pound-Drever-Hall locked to sample cell, finesse >15 000– power at cell input ~500 mW up to 1000 W inside– near absorption peak center, small de-tuning

• Probe beam– OPO operates near 3300 nm / 3000 cm-1

– single pass through sample cell at 0.5° angle– detected with cooled InSb detector & lock-in amplifier– power initially ~300 mW <5 mW at sample cell input– scanned across stimulated emission at 0.05 cm-1/50 s

Typical measurement data[21+]

100 200 300 400 500 600 700 800 900

3059.36

3059.37

3059.38

3059.39

3059.40

3059.41

3059.42

Measurement time [s]

Wav

enum

ber

of th

e pr

obe

beam

[cm

-1]

-1.5

-1

-0.5

0

0.5

1

1.5

2

Stim

ulat

ed e

mis

sion

sig

nal [

V]

Two peaks due to the build-up cavity

• Two sub-Doppler peaks when the pump beam is slightly de-tuned from the absorption

• Pump light propagates in both directions in sample cell

• Peaks match the positive and negative Doppler shift due to pump beam de-tuning65 70 75 80 85

0.0

0.5

1.0

1.5

2.0

Stim

ulat

ed e

mis

sion

sig

nal (

arb.

uni

ts)

Scan time (s)

[21+]

Pump beam adjustment

S/N > 500FWHM < 0.0005 cm-1

Pump laser frequency

C2H2 absorptionPUMPPROBE

[30+]

Comparison to LIDF data[30+]

Wavenumber [cm-1]

Fluo

resc

ence

inte

nsity

[arb

. uni

ts]

29792980298129822983

NOTE THE HORIZONTAL RESOLUTION

This workExtract from earlier LIDF data

3034.6 3034.7 3034.80123456789

10

Wavenumber [cm-1]

Stim

ulat

ed e

mis

sion

[arb

. uni

ts]

Typical single ro-vibrational peaks [21+]

Overlay of many scans with varying amount of pump beam de-tuning

S/N > 10

FWHM of single peak < 0.0005 cm-1

Typical single ro-vibrational peaks [21+]

New results on C2H2 data[30+] center at 9663.362(16) cm-1

Standard deviation 5.44x10-2 cm-1

B=1.15780(18) cm-1, D=1.21(44)x10-6 cm-1

[21+] center at 9991.9725(13) cm-1

Standard deviation 1.37x10-3 cm-1

B=1.156145(23) cm-1, D=1.608(88)x10-6 cm-1

J’’ J’νpump /cm-1

(from literature)

νse /cm-1

(measured)

νOBS-νCALC /cm-1

5 3 13020.9999 3050.451 0.0008

5 5 13020.9999 3029.642 0.0009

9 7 13010.2227 3059.399 -0.0029

9 9 13010.2227 3020.096 -0.0008

10 8 13007.3986 3061.603 0.0004

10 10 13007.3986 3017.682 -0.0010

11 9 13004.5212 3063.797 0.0007

11 11 13004.5212 3015.255 -0.0004

13 11 12998.6127 3068.151 0.0018

13 13 12998.6127 3010.370 0.0008

15 13 12992.4925 3072.456 0.0003

15 15 12992.4925 3005.440 -0.0008

J’’ J’νpump /cm-1

(from literature)

νse /cm-1

(measured)

νOBS-νCALC /cm-1

11 11 12699.9470 3039.075 -0.0088

13 11 12699.9470 2981.189 0.0066

14 12 12701.6430 2978.716 -0.0011

17 17 12709.4526 3051.821 -0.0004

12 10 12698.2069 2983.673 -0.0031

10 10 12698.2069 3036.911 0.0026

9 9 12696.4128 3034.743 0.0011

11 9 12696.4128 2986.125 0.0030

Earlier LIDF/LIF data: 9663.3860 (11) cm-1

[M. Metsälä, S. Yang, O. Vaittinen, and L. Halonen, J. Chem. Phys. 117, 8686 (2002)]

Equation: E/hc = G + BJ(J+1) – DJ2(J+1)2 + …

Summary

• Access to symmetric

vibrational states in the

ground electronic state

• Sensitivity superior to LIDF

• Sub-Doppler resolution

• No background level needs

to be measured

• Vibrational state [21+]

(21+3) of C2H2 is

determined

Acknowledgement: The Academy of Finland for funding