The higher rydberg states of formaldehyde

Volume 47. number 2

CHEMICAL PHYSICS LETTERS

1.5 April 1977

THE HfCHER RYDBERG STATES OF FORMALDEHYDE

CR. DRURY-LESSARD and DC. MOULE Cepcrtmenr of Chemistry, Brock University, St. Cathatines, Ontario, Canada Received 1 October 1976

Tire assignment of tbc vacuum uitrevioiet transitions in formaldehyde below 1500 A is reexamined. Rydbeg series involvin: transitions from the nonbonding n orbital to s, p,,, pr, d and f orbinds are assigned in the spectrum_ The fust ioniaation potentials in HlCO and DsCO are reevaluated from the spectra1 data to be 10.874 f 0.002 eV end IO.901 -+0.006 eV respectively.

I, Introduction

2. Experimental

In a previous paper [I] we have studied the vacuum ultraviolet spectrum of H,CO, HDCO, D,CO and DXi3C0 in the region 1600 to 1300 A. Under conditions of moderateIy high resolution, it was found that the transitions to the states characterized by the principal quantum number n = 4 have a vibrational fme structure which correlates closely with the vibrational structure which is observed in the first system of the H,CO photoelectron spectrum. This is to be expected if?he diameter of the partiahy filled Rydberg MO is sufficientfy large that the electron does not interact with the ionic co-e and stabilize the bonding in the molecule. The Rydberg bands which he at Ionger wavelengths which have been assigned to the n = 3 quantum states, however, are quite different in appearance from the photoelectron spectrum and from the n = 4 Rydberg systems. From the hydrogen-deuterium isotope effect and the vibrational activity of “r, v2 and v3 in the spectrum it was inferred that the molecular structures of these low energy states are not like those of the ionic state. When the region below 1300 & is examined, many absorption features are observed which can be fitted into Rydberg series and followed out to the series limit. There are. however, bands in this region which have very sharp rotational features which do not fit into the pattern established by the n --f 3s, n +3py, n-t 3pz or n + 3d transitions. It is the purpose of this paper to make an assignment for these bands.

The spectra were recorded on a McPherson 3 meter concave grating spectrograph which was operated in the first order. With an 1100 grooves mm-’ grating, the plate dispersion was 2.7 A mm-I _The sample of formaldehyde, D,CO, was obtained from Merck, Sharpe and Dohme in the form of solid paraformaldehyde. To liberate the monomer, a small amount of paraformaldehyde was placed in a sidearm which was attached

300

directly to the absorption

cell and was gently warmed.

Some difficulty was encountered in the formation of a white polymer on the MgFz windows of the cell and

it was necessary to frequently dismantle and cIean the external optical train. Spectra were also recorded with a 10.7 meter concave grating spectrograph which has been described by Douglas and Potter [2] _ The spectra were photographed in the first order of a 1200 grooves mm-l grating. In this experiment the formaldehyde was confined in the absorption tube by two streams of argon, one flowing away from the source and the other from the spectrograph slit.

3. Discussion Fig. 1 shows the absorption spectrum of H,CO in the vacuum ultraviolet region from 1245 A down to 1200 A. The bands which occur in the longer wavelength regions at 1242,1223 and 1209 A fit into the

Vol iume

47, number 2

15 Apti :97-f

CHEMICAL PHYSICS LETTERS

H,CO

---

D,CO

t

n - 5d

n - 5s

i

n - 4f i

Fig. I. Absorption spectrum of HzCO and D2CO from t 245 to I200 A.

pattern expected for the n = 5 members of the Rydberg series and can be assigned without difficulty to the n -t I%, n + 5p and n --+Sd orbital promotions. In this region, there is a baud at 1237 8, which possesses a well defined rotational structure which cannot be accounted for by a similar extrapolation of the existing Rydberg series. The assignment of this band poses some difficulty. With a transition energy of 10.0 eV, the position of the band is very close to the energy predicted for the srst member of the Rydberg series which terminates on the second series limit, 1 Bt(n, 3s) + % f A, _The absence of a progression in v2, the CO stretching mode, suaests that the promoted electron does not arise from the strongly bonding K orbital, which precludes this assignment. From fig. 1 it is clear that the displacement of the bands on deuterium substitution is in the same direction and is of the same size and magnitude as the adjacent bands assigned to the n * Sd and n -+ Sp tram& tions. This info~at~on then provides the key to the identity of the electron promotion responsible for the transition- As the Sd and 5p MO’s do not contribute to the bonding in formaldehyde, the isotope effect on

the origin band must be associated with the structural. changes which result from the loss of the R nonhanding electron. The 1237 a band can he assigned to an eIectron promotion from a lower nonbon~ng n orbital to a Rydberg MO which is sufficiently large that it avoids the ionic core. This band therefore must be a member of a Rydberg series which terminates on tie First ionization limit. The shift to higher frequencies on deuterium substitution identifies the system as a Q-4 transition. If a principal quantum number ofn = 4 is selected for the Rydberg MO and Chupka’s [3] recent value of 87659 cm-l is used for the ionization potential, then ER = 87659 - Rj(it - O.O3)2, where the Rydberg constant R = ‘109737 crnMi. As the S values for the 4s, 4p,, and 4d states are I _tCd,O.76 and 0.37, the value observed here of 0.03 must be reconciled with a MO with hi& orbital angular momentum. The lack of vibrations fme structure, the very small quantum defect, and the consideration that the

appearance of this band is different from that of any Rydberg band to lower frequency, suggests the assign301

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CHEMICAL PHYSICS LETTERS

ment of the upper state to an f Rydberg MO, and hence the transition can be designated as n +4f. With this assignment, it became possible to locate a new series in the spectrum which could be followed out to IZ = 12 before it merged into the background absorption near the series limit. The assignment of the bands in H,CO and D,CO and their frequencies are collected together in table 1. The assignment of a Rydberg series to electron pro-

15 April 1977

motions which involve the population of an f type Rydberg MO is unusual for a polyatomic molecule. Greening and King [4] have observed in the vacuum UV spectrum of CSe,, a series of bands which they assign to ‘iT+ f electron promotions. The intensity of this system they attribute to the shape of the lower n MO which in the united atom approximation takes the form of a d type AO. A transition from this state to an f state wouId then be made allowed by the Al = + 1 seIection

73btc I Obccrvtd frequcncics _-_

for the bands in the first Rydbcrg series in HzCO and DzCO, in cm-’ Assignment

H2C0 frequency

n + 3s

D*CO a)

frequency

6 b,

57491

1.10

644 73

0.83 0.77

57180 64276

1.11 0.84

65584 71598 74692 77278

0.39 1.10 0.76

65776 71683 74835 77406

0.40 1.10 0.76

85 163 128

77646 79378 80820 80503

0.70 0.37 0.01

77760 79437 80974

0.70 0.39 0.00

114 59 154

1.10

80629

0.71 0.32 0.07 0.69 0.29 0.06

81842 82788 83443 a4001 84413

1.10 0.73

I26 93

6~ 6d 6f

81749 82695 83197 83814 84347 84602

0.34 0.01 0.66 0.35

93 246 187 66

7P 7d

84958 85364

0.69

7f

85417 85686 86024

0.09 0.65 -0.05

86224

0.42

86348 86487 86634 86736 86809 86952

0.04 0.55 -0.07 0.42 0.00 0.02

3pY 3PZ 3d 4s 4PY 4PZ 4d 4f

5s 5PY Sd 5f

8~ 8f 9P 9f IOP 10f 1lP llf 12f

‘) Accuracy is + 15 cm -t for HzCO and +SO cm-’ for DzCO. b) VR = tP -- Rl(n - 6)2_ =) A = v(D2CO) - v(H2CO).

302

6 b)

AC)

0.77

0.17

3il 197 192

Volume 47, number 2

15 April t977

CJJEMICALPHYSICSLETPERS

rule. A similar rne~h~i~ also could explain the appearance of the f Rydberg transitions in fo~~dehyde. Recent MO calculations [S] have shown that the nonbonding orbital in formaldehyde has lobes directed along the CH bonds of gerade parity with respect to the lobes of electron density projecting out on either side of the oxygen centre. That is, the n MO has the form of a d type A0 in the united atom approximation. The f type Rydberg series would then result from the atomic selection rule for the angular momentum which would ailow that part of the n MO which contains d character to combine with the f Rydberg orbital through electric dipole radiation. The ionization potential for H2C0 was determined from a least squares analysis of the new Rydberg series, for which nine members can be identified. The best fit to the data was for a quantum defect of 6 = 0.03. From this value the ionization potential for D,CO was calculated. Values of IO.874 + 0.002 and 10.901 t- 0.006 were derived for H,CO and D2C0 respectively. These are to be compared to Chupka’s values of 10.868 * 0.0

and 10.882 + 0.005 eV respectively.

Acknowledgement The authors would like to express their indebtedness to Dr. S. Bell for assistance with the low resolution work. Thanks are also due to Dr. A.E. Douglas for permission to use the 35 foot vacuum spectrograph and Mr. F. AIberti for carrying out the high resolution experiments.

References [ 11 C.R. Lessardand D.C. hfoule, J. Chem. Phys.,_to be published. [2] A-E. Douglas and J.G. Potter, Appl. O+ L (1962) 727. 131 P.h-i.Guyon, W-A. Chupka and J. Berkowitz, J. Chem. Phys. 64 (1976) 1419. [4] P.R. Greening and G.W. King, J. S%o?. Spectry. 6t (1976) 459. f5] M.K. O&off and

N.B. Coltttup, 3. Chem. Educ- 50 (1973)

400.

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