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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
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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
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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)]
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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
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C2H2 pump beam absorption
[31-] in local mode notation [40-] in local mode notation [L. Halonen, Adv. Chem. Phys. 104, 41 (1998)]
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Transitions used in the experiments
[40-] (1+3 3) cm-1 [30+] (1+23) cm-1 [00+] 0.0 cm-1 State Probe, ΔJ = ±1 Pump, ΔJ = +1 [31-] (31+ 3) cm-1 [21+] (31) cm-1 [00+] cm-1 Probe, ΔJ = ±1 State Pump, ΔJ = -1
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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
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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
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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]
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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+]
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Pump beam adjustment PUMP PROBE [30+] C2H2 absorption
S/N > 500 FWHM < cm-1 Pump laser frequency C2H2 absorption PUMP PROBE [30+]
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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
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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
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Typical single ro-vibrational peaks [21+]
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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 + …
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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
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