ROTATION-VIBRATIONAL ANALYSIS OF THE BANDS OF FORMALDEHYDE FALLING IN THE 3900 TO 5300 CM-1 REGION

W.J. LAFFERTYOptical Technology Division

NISTGaithersburg, MD 20899

J.-M. FLAUDLISA

CNRS/ Univ. Paris 12 et 761 Av. du General de Gaulle

94910 CreteilFRANCE

R.L. SAMS AND S.W. SHARPEPacific Northwest Laboratory

P.O. Box 999Richland, WA 99352

Background

* Formaldehyde lines have been observed in urban atmospheres as well as over forest fires.

* In this work, we discuss the spectrum of CH2O in the region

3900 to 5100 cm-1.

* This overtone region contains a myriad of interactingbands linked by anharmonic and Coriolis resonances.

* The region has been previously studied by T. Tiptonet al. We have expanded their study greatly.

* At this point 8 bands have been assigned and fit giving a

standard deviation of 0.004 cm-1.

Molecular details

• C2v symmetry• Spin statistics: Ka even 1; Ka odd 3• Lowest vibration at ~ 1200 cm-1

• A=9.41 cm-1 ; (B+C)/2 = 1.215 cm-1

• Large centrifugal distortion effects• About 30 vibrational levels in the region• Every observed band perturbed and a

challenge to assign

EXPERIMENTAL DETAILSBruker IFS 120 HR Fourier transform spectrometer.

Resolution 0.0030 cm-1

16 meter optical path.

Pressures ranging from 0.1 Torr to 0.6 Torr.

Calibration with CO2 lines in spectrum.

Very Empirical Treatment of the 1161 State The ν1 + ν6 band is globally perturbed. In addition there are local perturbations The J2 J-2 levels are strongly perturbed. The low Ka levels can not be fit satisfactorily. Using symmetry arguments the 1161 state must be in an A-type Coriolis resonance with the 1141

levels. The J2 J-2 levels must have a B-type Coriolis interaction with the J3 J-2 levels of the nearby 4151 state. The low Ka levels weakly interact via a C-type interaction with 2142. The frequencies of the dark states are known fairly well thanks to ab initio calculations deriving the quartic force field as well as dispersed fluorescence studies. By floating the vibration frequencies and few rotational constant of the dark states, one obtains a fit within a factor of two or three of the experimental uncertainty.

1141

4151

1161

2142

1141

HW

4151 CC

HW

1161 CA

CB

HW

2142 0

0 CC

HW

HW = Watson Hamiltonian

xyzA

xyzA

zA

xyA

zAA JiJJhJJhJhJiJhJhC ,,,, 2

52

42

321 J x

Bzy

Bx

BB JhJiJhJhC 2321 , J

yC

zxC

yCC iJhJJhiJhC 2

321 , J

BAABBA , and 2 2 2J J Jxy x y 222yxxy JJJ

INTENSITY MEASURMENTS

The intensities of 20 to 30 lines of each assigned band were measured using the Origin line fitting program over a range of quantum numbers. Due to uncertainties in the CH2O pressure, the absolute intensity uncertainty is about 20% although relative intensities are much better.

Because of resonance mixing effects the intensity measurements serve as a check on the assignment as well as the fitting.

The band intensities of 7 bands have been obtained.

A comprehensive list of line positions and intensities of the 4 strongest bands has been produced

Assigned bands and band intensities

Band Band center

(in cm-1)

Band intensity at 296K

10-19cm-1/(molecule cm-2)

16 4021.082 0.0145

ν2+ν4+ν5 4162.423 forbidden

3 +6 4192.382 0.0327

3 +5 4335 .096 0.0966

12 4529.502 0.2431

25 4571.695 0.3772

26 4734.208 0.1081

2 5177.759 0.0568