Observation of combination bands involving intermolecular vibrations of CO 2 -, N 2 - and OCS-N 2 O complexes using an external cavity quantum cascade laser M. Rezaei, S. Sheybani-Deloui, N. Moazzen-Ahmadi Department of Physics and Astronomy, University of Calgary A.R.W. McKellar National Research Council of Canada
External Cavity design Linewidth: 45 MHz Power: mW Full tuning range: cm -1 Mode Hop Free range: cm -1
No need for cryogenic cooling High power ( mW) Better reproducibility Wide tuning range (~100 cm -1 ) Mechanically tuned by PZT attached to grating Prone to mechanical noise Wavenumber agility is less than that of a diode laser. Higher 1/f noise
Jet Trigger 12 bit DAQ Card Timer Controller Card (CTRO5) DAQ Trigger Gas Supply Jet Signal IR Detectors Etalon QCL PZT Controller Function Generator Valve Controller (Iota One) Ref. Gas Pump Frequency Locking Box Supersonic Expansion Nozzle
Time sequence for data acquisition (QCL)
reference gas signal boxcar gates Reference gas cell Sweep stabilization using 2-channel boxcar 2-channel boxcar integrator SR250 ab PZT controller output(b a) oscilloscope De Piante, Campbell, & Buelow, Rev. Sci. Instr. 60, 858 (1989) TTL Function generator
Averaged spectrum Jet signal (subtracted background) Reference gas signal Etalon signal Averaged jet signal
Current etalon signal Current etalon signal Comparing peaks Averager/Shifter Reference etalon signal shift Werle, et al., Spectrochimica Acta Part A 60 (2004) 1685–1705
Jitter suppression off Jitter suppression on Enhanced resolution!
T-shaped structure for N 2 -N 2 O dimer Howard et al., Faraday Discuss. Chem. SOC., 86 ( 1988), 21. Slipped parallel structure for CO 2 -N 2 O dimer Dutton et al, J. Phys. Chem., 100 (1996) Two isomers of the OCS-N 2 O complex H.O. Leung et al., 114 (2001) M. Afshari et al., CPL 489 (2010) 30.
Torsional conbination bands for N 2 O-CO 2 M. Afshri et al., JCP 129 (2008) Intermolecular modes do not depend strongly on the intramolecular vibrations in the weakly bound van der Waals complexes.
Resulting intermolecular frequency cm -1 We used the PGopher computer program for assignment, simulation, and fitting of the spectra. (
cm -1
Torsional frequency for isomer b17.11 cm -1 CO 2 -OCS isomer a18.73, cm -1 CO 2 -OCS isomer b 15.9 cm -1 C-type band
T-shaped structure for N 2 -N 2 O dimer Howard et al., Faraday Discuss. Chem. SOC., 86 ( 1988),21 b-type band A potential based on distribute multiple and distributed dispersion model H.O. Leung, JCP 110 (1999) Mw observation of 15 N 2 -N 2 O The 15 N 2 axis and the N 2 O axis make an angle of 13° and 81° with the intermolecular axis, respectively. a/b-type band with a weak a-type component
a/b-type band with a weak b-type component An in-plane intermolecular vibration and definitely not out-of-plane torsion. Intermolecular vibrational frequency of cm -1
Successful implementation of a CW-EC QCL in our apparatus with Rapid-scan signal averaging technique Continuous background subtraction Laser frequency locking to reduce long term drifts Post detection signal processing to reduce laser jitters Observation of combination bands of CO 2 -, OCS- and N 2 -N 2 O (in 1 fundamental region of N 2 O).