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

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

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)]

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+23) 9 663.36 cm-1 [00+] 0.0 cm-1 State Probe, ΔJ = ±1 Pump, ΔJ = +1 [31-] (31+ 3) 13 033.30 cm-1 [21+] (31) 9 991.97 cm-1 [00+] 0.0 cm-1 Probe, ΔJ = ±1 State Pump, ΔJ = -1

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

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] Wavenumber of the probe beam [cm-1] -1.5 -1 -0.5 0.5 1 1.5 2 Stimulated emission signal [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-tuning [21+]

Pump beam adjustment PUMP PROBE [30+] C2H2 absorption S/N > 500 FWHM < 0.0005 cm-1 Pump laser frequency C2H2 absorption PUMP PROBE [30+]

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

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 11 12699.9470 3039.075 -0.0088 13 2981.189 0.0066 14 12 12701.6430 2978.716 -0.0011 17 12709.4526 3051.821 -0.0004 10 12698.2069 2983.673 -0.0031 3036.911 0.0026 9 12696.4128 3034.743 0.0011 2986.125 0.0030 J’’ J’ νpump /cm-1 (from literature) νse /cm-1 (measured) νOBS-νCALC /cm-1 5 3 13020.9999 3050.451 0.0008 3029.642 0.0009 9 7 13010.2227 3059.399 -0.0029 3020.096 -0.0008 10 8 13007.3986 3061.603 0.0004 3017.682 -0.0010 11 13004.5212 3063.797 0.0007 3015.255 -0.0004 13 12998.6127 3068.151 0.0018 3010.370 15 12992.4925 3072.456 0.0003 3005.440 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+] (21+3) of C2H2 is determined Acknowledgement: The Academy of Finland for funding