Kirill Kuyanov-Prozument AnGayle Vasiliou G. Barratt Park John S. Muenter John F. Stanton G. Barney...
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Transcript of Kirill Kuyanov-Prozument AnGayle Vasiliou G. Barratt Park John S. Muenter John F. Stanton G. Barney...
Vibrational Population Distribution in Formaldehyde
Expanding From Chen Pyrolysis Nozzle Measured by
Chirped-Pulse Millimeter-Wave Spectroscopy
Kirill Kuyanov-ProzumentAnGayle VasiliouG. Barratt ParkJohn S. MuenterJohn F. StantonG. Barney EllisonRobert W. Field
Massachusetts Institute of Technology
University of Colorado, Boulder
University of Rochester
University of Texas
Molecular Spectroscopy SymposiumOhio State University
June 21, 2011
Thanks: Brooks Pate and Justin Neill, University of Virginia
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
supersonic expansion
General Valvepulsed nozzle
SiC tube: 300 – 1750 K(Tubular Reactor)
Residence time: 65 μs
Chirped-Pulse Millimeter-Wave (CPmmW) – Pyrolysis Experiment
Tubularreactor
source hornreceiving horn
Teflon lens
74.91 GHz
νmixer
Fast oscilloscope
×8
10.7 GHzAWG
12 cm
L ≈10 cm
5 ×10-5 mbar
mixer
CP spectroscopy: rapid acquisition of broadband high resolution spectra1), 2)
1) Microwave: Brown et al, RSI 79, 053103, 20082) Millimeter-wave: Park, Steeves et al, accepted to JCP, 2011
Free Induction Decay (FID)Chirped Pulse
Thermal Decomposition of Methyl Nitrite
H3CO
NO
H3CO
H2CO
H
71.5 72.0 72.5 73.0Transition frequency, GHz
CH3ONO
H2COVib. ground state
H2CO
ν4
H2CO
2ν4
H2CO
3ν4
H2CO
ν2
101 – 000 rotational transition
Neat Formaldehyde Expansion
71.0 71.5 72.0 72.5 73.0
Transition frequency, GHz
Vib. ground stateH2CO
ν4
H2CO
2ν4
H2CO
3ν4
H2COν2
H2COVib. ground state
H213CO
ν3
H2CO
Tnozzle ≈ 1750 K
Formaldehyde2782 cm-1
1167 cm-1
72 492.56 MHz
1500 cm-1
73 062.63 MHz
2843 cm-1 1249 cm-1
72 727.25 MHz
1) vib. frequency2) 101 – 000 frequencyin particular vib. mode
1746 cm-1
72 348.42 MHz
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
Formaldehyde modes from: Bouwens et al, JCP 104, 460 (1995)
Collisional Relaxation in FormaldehydeTnozzle ≈ 1750 K, Tvib – shown
0
2000
3000
Evib, cm-1
1000
ν1 ν3 ν6ν5ν4ν2
ν2 + ν3
ν2 + ν4
ν2 + ν6ν3 +
ν4
ν3 + ν6ν4 +
ν6
k1
k2
k3
500 K420 K
1100 K < 320 Kk3 > k2 > k1 A-type Coriolis interaction
940 K
940 K
OCS: vibrationally hot
72.5 73.0 73.5Transition frequency, GHz
000
100
110, 1e 110, 1f
010, 1e010, 1f
020
020, 2f, 2e 030, 3f, 3e030, 1f
Tnozzle ≈ 1750 K
Tvib = 600 –800 K
Acetaldehyde: vibrationally cold
76.5 77.0 77.5
Transition frequency, GHz
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
Tnozzle ≈ 1750 KTrot ≈ 8 K
77.08 77.09
4 04–303
4 31–330
4 23–322
4 22–321
4 22–3214 23–322
4 32–331
4 31–330
4 32–331
Each rot. transition is split due to internal rotor
Conclusions
Vibrational population distribution is highly molecule- and mode- dependent. Three cases studied:Formaldehyde: highly mode-specific vibrational temperatureOCS: unrelaxed vibrations with Boltzmann population distributionAcetaldehyde: Effective vibrational relaxation (internal rotor)
The Chirped Pulse technique is an excellent tool for studying the vibrational population distribution
FutureRelaxation in large molecules without low-lying statesVibrational relaxation of acetylene local bender – vinylidene states
71.0 71.5 72.0 72.5 73.0
Transition frequency, GHz
Neat Formaldehyde ExpansionVib. ground state
H2COν4
H2CO
2ν4
H2CO
3ν4
H2CO ν2
H2CO
Vib. ground stateH2
13CO
×20 times
ν3
H2CO
Tnozzle ≈ 1750 K