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