Molecular Spectroscopy Symposium Vibrational Population Distribution in Formaldehyde Expanding From Chen Pyrolysis Nozzle Measured by Chirped-Pulse Millimeter-Wave Spectroscopy Kirill Kuyanov-Prozument AnGayle Vasiliou G. Barratt Park John S. Muenter John F. Stanton G. Barney Ellison Robert W. Field Massachusetts Institute of Technology University of Colorado, Boulder University of Rochester University of Texas Thanks: Brooks Pate and Justin Neill, University of Virginia Molecular Spectroscopy Symposium Ohio State University June 21, 2011

Motivation Collisional energy transfer mediates chemical reactions Supersonic expansion from a hot nozzle: temperature drop from 1750 to 1 K Case study: Formaldehyde, OCS and Acetaldehyde Vibrational relaxation – a slow process. How general that rule is? Are there molecules that behave badly?

Chen Pyrolysis Nozzle: Ellison Group Model optical pyrometer General Valve pulsed nozzle SiC tube: 300 – 1750 K (Tubular Reactor) Residence time: 65 μs supersonic expansion

Chirped-Pulse Millimeter-Wave (CPmmW) – Pyrolysis Experiment Teflon lens L ≈10 cm receiving horn 12 cm source horn Tubular reactor mixer ν 74.91 GHz 5 ×10-5 mbar 10.7 GHz Fast oscilloscope AWG ×8 mixer Free Induction Decay (FID) Chirped Pulse CP spectroscopy: rapid acquisition of broadband high resolution spectra1), 2) 1) Microwave: Brown et al, RSI 79, 053103, 2008 2) Millimeter-wave: Park, Steeves et al, accepted to JCP, 2011

Thermal Decomposition of Methyl Nitrite H3C N H3C H2C H2CO ν4 H2CO Vib. ground state 101 – 000 rotational transition H2CO ν2 H2CO 3ν4 H2CO 2ν4 CH3ONO

Neat Formaldehyde Expansion Tnozzle ≈ 1750 K Vib. ground state H2CO ν4 H2CO 3ν4 H2CO ν2 H2CO ν3 H2CO Vib. ground state H213CO 2ν4 H2CO

Formaldehyde 2782 cm-1 1746 cm-1 72 348.42 MHz 1500 cm-1 73 062.63 MHz 1) vib. frequency 2) 101 – 000 frequency in particular vib. mode Vib. g. s. 101 – 000: 72 837.948 MHz A = 281 970.57 B = 38 836.05 C = 34 002.20 µa = 2.3315 D 2843 cm-1 1249 cm-1 72 727.25 MHz 1167 cm-1 72 492.56 MHz Formaldehyde modes from: Bouwens et al, JCP 104, 460 (1995)

Collisional Relaxation in Formaldehyde Tnozzle ≈ 1750 K, Tvib – shown Evib, cm-1 ν2 + ν3 940 K ν2 + ν6 3000 ν2 + ν4 ν3 + ν6 ν3 + ν4 ν4 + ν6 940 K 2000 k1 500 K k2 420 K 1000 1100 K k3 < 320 K k3 > k2 > k1 A-type Coriolis interaction ν1 ν2 ν3 ν4 ν5 ν6

OCS: vibrationally hot Tnozzle ≈ 1750 K 000 Tvib = 600 –800 K 010, 1e 010, 1f 100 020, 2f, 2e 030, 3f, 3e 110, 1e 110, 1f 020 030, 1f

Acetaldehyde: vibrationally cold Tnozzle ≈ 1750 K Trot ≈ 8 K Mode ν, cm-1 Tvib, K CH3 a str 3014 < 693 C=O str 1746 < 402 C–C s str 866 < 199 CH3 rock 764 < 176 C=O bend 509 < 117 CH3 torsion 144 < 33 4 04–303 4 32–331 4 32–331 4 31–330 4 31–330 4 22–321 Each rot. transition is split due to internal rotor 4 23–322 4 23–322 4 22–321

Conclusions Vibrational population distribution is highly molecule- and mode- dependent. Three cases studied: Formaldehyde: highly mode-specific vibrational temperature OCS: unrelaxed vibrations with Boltzmann population distribution Acetaldehyde: Effective vibrational relaxation (internal rotor) The Chirped Pulse technique is an excellent tool for studying the vibrational population distribution Future Relaxation in large molecules without low-lying states Vibrational relaxation of acetylene local bender – vinylidene states

Neat Formaldehyde Expansion ν4 H2CO Vib. ground state H2CO Tnozzle ≈ 1750 K 2ν4 H2CO ×20 times 3ν4 H2CO Vib. ground state H213CO ν2 H2CO ν3 H2CO