1. 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, Finland
2Centre for Metrology and Accredation, Espoo, Finland

2. In short Pump-probe experiment Sample is acetylene, C2H2
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

3. 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
lisää ref, lisää FTIR seliteteksti, muista symmetry, 1-photon
[M. Metsälä, S. Yang, O. Vaittinen, and L. Halonen, J. Chem. Phys. 117, 8686 (2002)]

4. 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

5. C2H2 pump beam absorption
[31-] in local mode notation
[40-] in local mode notation
[L. Halonen, Adv. Chem. Phys. 104, 41 (1998)]

6. Transitions used in the experiments
[40-] (1+3 3)
cm-1
[30+] (1+23)
cm-1
[00+] 0.0 cm-1
State
Probe, ΔJ = ±1
Pump, ΔJ = +1
[31-] (31+ 3)
cm-1
[21+] (31)
cm-1
[00+] cm-1
Probe, ΔJ = ±1
State
Pump, ΔJ = -1

7. Sample cell setup (simplified)
Pump beam from
Ti:sapphire laser
Acousto-optic
modulator
Measured
signal
C2H2 pressure
0.05 – 0.5 Torr
Gas
inlet
PDH lock
feedback
electronics
Lock-in
amplifier
Photo-
diode
Pressure
meter
Beam-
splitter
Dichroic
beam
splitter
1/4-wave
plate
LN2-cooled
InSb detector
Probe beam
from mid-infrared optical
parametric oscillator (OPO)
Highly reflective mirrors
at probe beam frequency
Ring piezo
actuator
Oscilloscope
Photo-
diode

8. Measurement setup properties
Pump beam
cw Ti:sapphire near 800 nm / cm-1
chopped at 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

9. Typical measurement data [21+]
100
200
300
400
500
600
700
800
900
Measurement time [s]
Wavenumber of the probe beam [cm-1]
-1.5 -1
-0.5
0.5 1
1.5 2
Stimulated emission signal [V]

10. 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-tuning
[21+]

11. Pump beam adjustment PUMP PROBE [30+] C2H2 absorption
S/N > 500
FWHM < cm-1
Pump laser frequency
C2H2 absorption
PUMP
PROBE
[30+]

12. Comparison to LIDF data [30+]
Extract from earlier LIDF data
This work
Wavenumber [cm-1]
Fluorescence intensity [arb. units]
2979
2980
2981
2982
2983
3034.6
3034.7
3034.8 1 2 3 4 5 6 7 8 9 10
Wavenumber [cm-1]
Stimulated emission [arb. units]
NOTE THE HORIZONTAL RESOLUTION

13. 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 < cm-1

14. Typical single ro-vibrational peaks [21+]

15. New results on C2H2 data [30+] center at 9663.362(16) cm-1
Standard deviation 5.44x10-2 cm-1
B= (18) cm-1, D=1.21(44)x10-6 cm-1
[21+] center at (13) cm-1
Standard deviation 1.37x10-3 cm-1
B= (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 11 13
0.0066 14 12 17 10
0.0026 9
0.0011
0.0030
J’’ J’
νpump /cm-1 (from literature)
νse /cm-1 (measured)
νOBS-νCALC
/cm-1 5 3
0.0008
0.0009 9 7 10 8
0.0004 11
0.0007 13
0.0018 15
0.0003
Earlier LIDF/LIF data: (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 + …

16. 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+] (21+3) of C2H2 is determined
Acknowledgement:
The Academy of
Finland for funding