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Imine (NH) detected by laser magnetic resonance
Volume
34, number
CIIEMICAL
3
1 Au~st 1975
PHYSICS LETTERS
IMINE (NH) DETECTED BY LASER MAGNETIC RESONANCE H.E. FUDFOEUI and M.M. LITVAK Center forAstrophysics. arid Smithsoriti
ffafvard College Observatory. dsfrophyskal Observatory, Cambridge,
Massachusetts
02138,
USA
Received 13 hlay 1975
The N= l+ 0 rotational spectrum of tie free NH radical in its ground ‘C- state has been meaxurcd with an opticallypumped laser mgnetic resonance spectrometer, operating at wavelengths nest 0.3 mm. The hyperfine structure constants bN = -17.3 2 1 hIHz, CN = 107.4 C 6 MHz, bH = 15.9 * 1 hiHz and cH = 150.8 f 8 MHz ue derived from the spectrum.
1. Lntroduction
The gas-phase reaction of atomic
fluorine
with
am-
mania has been found recently to generate enough amino radical (NH2) to be detectable by its laser magnetic resonance spectrum [ 11. Using an improved ver-
sion of this spectroscopic method, at wavelengths near 0.3 mm, we have now detected the imine radical (NH) in the. same reaction. Although the optical spectrum [2] of gaseous NH and the near-infrared spectrum ]3,4] OFNH frozen in solids are we!1 known and thoroughly studied, this is the fust detection of the submillimeter rotational spectrum of NH.
2. Spectrometer The spectrometer
and spectrum consists
of an optically-pumped
submillimeter gas laser, 3 inches in diameter and 6 feet long, one end of which is partitioned off by a Brewster-angle window to form an intracavity absorption cell. This section of the laser tube occupies tI;e air gap of a 1Zinch laboratory electromagnet, and it also forms the reaction vessel of a flow@g gas reaction systern. Absorption of submillimeter power by the reacting gas mixture, at field strengths that correspond to magnetic resonance, is detected by a gold-doped germznium bolomeier, cooled to 2 K, which monitors the output
power of the laser. The optical pump is a grating-tuned CO2 laser, with 3 maximum output power of 20 W.
With variouslaser gases and CO, pump lines, more than 100 cw submillimeter laser lines can be generated, at wavelengths between 40 pm and 2 mm IS]. Of
these lines about 30, thoss with pump power thresholds below 5 W, can be used in our present magnetic resonance spectrometer; pump beams of higher power puncture the polypropylene film which forms the Brewster-angle window. The pump beam enters the spectrometer through a 2-mm diameter hole in one of the end mirrors, and the same hole is used to couple out submillimeter laser power to the,cooled bolometer detector. In other respects, the present spectrometer is much like those used in the earlier work on NH, [ 11, HO, [6] and CH [7], w!dch were excited by a dc electric discharge in the laser gas. The advantage of optical excitation over electrical excitation lies in the much larger number and wider wavelength distribution of laser lines that can be generated. This is an important consideration for’a spectroscopic technique that relies on near-coincidences of laser lines with moiecular spectral lines. Strong NH, absorption spectra, at many different laser wavelengths, are observable when CF, (Freon-14j gas is passed through a microwave discharge and mixed wi’& ammonia in the absorption cell at a total
pressure of about 0.2 torr, the mixture being adjusted for the brightest green chemiluminescence. The new NH absorption spectra appear &en the ammonia flow is reduced to the point where the chemiluminescence turns orange. Since the products of an electric dis-
Volume 34; number 3
chirge in CFq are known to contain atomic fluorine, an efficient abstractor of hydrogen, the reaction path is probably hydrcgen abstraction from NH3 to form
NH,, followed by a-second hydrogen abstraction, under conditions of excess atomic fluorine, to form NH. To verify-that
a carbonaceous CF, discharge was not responsible
product of the for the new spec-
tra, SF, gas, another known source of atomic fluorine, was substituted. The same spectra appeared, although with somewhat less intensity. Representative spectral lines recorded under the onngechemiluminescence conditions are shown in fig. 1. Only two of tk thirty-odd usa‘ulc laser wave-
length&302 Drn and 3 15 Dm, gave spectra that could not @ identified with NH2 or OH. The conventional Zeeman notation TIand u is used to label lines which appear when the subznillimeter
parallel or perpendicular two
difi’erent
1 Ausst
CHEMICAL PHYSICS LETTERS
electric
field is oriented
to .Lhemagnetic field. (The
field orientations
are produced
by rota-
1975
ting the Brewster-angle window about the axis of the laser tube.) The line profiles show the usual fiistderivative shape produced by magnetic modulation and synchronous detection. Line positions were measured to an accuracy off 0.01% with 2 nuclear magnetic resonance probe, inserted cell at the center of the magnet
into the absorption gap.
The 302 urn and 315.~rn laser lines are both pumped by the 9.6 pm R(4) line of CO,, the laser gases being formic acid and methyknine, respectively. We measured the two laser frequencies to a precision of 1 part in lo5 by comparing them with harmonics of a free-running millimeter-wave klystron, the frequency of which was in turn compared with harmonics of a centimeter-wave multiplier chain. The submillimeter harmonic mixer was a gallium arsenide Schottky diode, kindly loaned for this purpose by H.R. Fetterman of the M.I.T. Lincoln Laboratory. The results
were
Y(302 flm) = 991779 5 IO hi!&, ~(315 pm) = 952185
where the uncertainty
+ 10 MHz,
is due to the frequency instab-
ility of the m’tiimeter-wave
klystron.
3. Analysis
(bl
The two laser lines kwithin the fine structure the N = i ~‘0 rotational transition of the ground
, 43i-J
I
45a5
of
(3Z-, v = 0) state of NH, as shown by the energy level diagram of fig. 2. The magnetic field dependence of tie ener,y sub-levels in this diagram was calculated as a perturbation of the zero-field energy levels by the
simple Zeeman operator
(cl. 302
d
where g, = 2.0023
is the electron spin g-factor. p. is the Bohr magneton, S is the electron spin angu!ar momentum and His the magnetic field. Only matrix. L
I
I
1554
1565
MAGNETIC FIELO STREiGM
IN GAUSS
Fig. 1. Repsentative line5 from the magnetic resonance s$xtru~ of IW, at wavelengths af 302 urn 2nd 315 Mm, in ~2rpencIiculir pohi&on. (a} 1”hH(3 C-, ,u I- 0); (t) %$pti--, P.+ !j;_(c) ‘wq3r-, Y = 0).
: ,;,.
:.,
.
elements diagonal in N were retained in the calculation. Zero-field energy levels derived from ‘Jle ultraviolet spectrum of NH were used in the drawing of fig. 2, and the same levels provided initial values of parameters fir an iterative computer fit of the spectra. Eyperfme structure was not included at this stage in the analysis; the rtiean magnetic field values of byper_..
CHEMICAL PHYSICS LETTERS
Volume 34, number 3
served, and again the two near 11 kG are not independent. With five independent data points to determine three parameters, there is enough redundancy to verify the quantum number assignments represented by fig_ 2 but hardly enough to warrant a more detailed theoretical treatment of the Zeeman effect. The values of the best-fit parameters are v(fV=1,5=2+h’0,J=1)=32.50.53
f 0.0005 cm-l
,
Av(N=l ,J=2-+1V=I ,J=O) = 0.9348 t 0.0005 cm-’
,
Av(N=1,J=1~~V=1,J=2)=0.8500~0.0005
which agree satisfactorily with the optical values shown in fig. 2. The assigned errors are estimates based on the uncertainty in the laser frequency meas-
b
i
m b
N’O
J=l
cm-1 ,
urements
and on approximations
made
in calculating
the Zeeman effect. The largest of the neglected contributions to the Zeeman effect are those of magnetic interactions between adjacent rotational levels, and the effects of orbital magnetism, each of which can shift the lines by an amount comparable to their total width of 4 MHz. The missing line of the 3 15 pm spectrum, shown dashed in fig. 2, is predicted to lie at 6827 G, in XI otherwise clear part of the spectrum. We have no convincing explanation for its absence, but it is probab!y
’
significant
that the intensities
of lines that are observed
Fy;.
agree poorly with intensities calculated for thermal equilibrium conditions in a-passive absorption cell. Further observations of the 0.3 mm spectra under different reaction conditions may resolve these intensity discrepancies. The hyperfine structure patterns of the Zeeman lines can be accounted for by computing expectation values of the magnetic dipole interaction operator,
fine structure patterns such as those of fig. 1 were taken as data points for the computer fit, together
Nhfs = (bNIN + bf.#l{) -S’+ (C&J + c&i)
I
0
m
5 MAGNETIC
FIELD
STRENGTH
I5 IN
KILOGAUSS
2. Encr,~ sublevels of the lowest two rotntiod levels of 14NH(3Z-, Y= 0) in a magnetic field. Solid YIOWSindicate the observed lines; the dashed arrow indicates an allowed transition that was not observed.
with the measured laser frequencies. TXe minimum number of parameters that is required for an adequate fit to the data is three, and we take them to be the zero-field frequency of the transition N= 1, J=2 + N= 0, J= 1, and the two fine structure separations of the zero-field hr= 1 levels. In the 302 Irm spectrum, all of the four predicted lines were observed, but the positions of the two 1 I kG lines are not independently determined by the three parmeters. Iti the 3 1.5pm spectrum, three of the faur predicted lines were ob-
-a-s
1 (2)
where k is the unit vector along the molecular axis and the subscript N refers to the nitrogen nucleus and H to the hydrogen nucleus. The coupling constants b and c are those defined by Frosch and Foley [8]- The admixed field-dependent wavefunctions necessary for computing expectation values are obtained as; a byproduct of the computer fit to,the Zeeman spectrum. The result of this czlculation, which was done by standard Spherical tensor methods [9], is an expression for the hyperfine pattern of each Zeeman line, linear ia 563
Volume 34, number
b,, CN, bx
and cH- The patterns of aUseven lines
identified by fig. 2 are found to be consistent with a single set of these i’our constants, and a least-squares fit gives the values bN = -17.3 b,
=.
5 1 MHZ,
15.9 2 1
MHz ~
cN=107.4+6MH.z,
cH = 150.8 58 MHz,
where the experimental uncertainty assigned to each fitted value is t‘hree times its standard error. This is meant to account for possible systematic error, introduced by our neglect of zlectric quadrupo!e hyperfine structure and intermediate field effects on the mag netic hyperfme structure. Both effects would cause irregdarities in the hyperfine patterns, and both were left out because the irregularities in the measured patterns were in no case greater than one half of the line width. In addition to the seven strong lines identified in fig. 2, all of which resemble fig. la, several weaker lines appeared in high-sensitivity field scans. Three lines at 3 15 pm, of which fig. 1b is representative, correspond closely to the strong 302 pm spectrum, and are consistent with ~1rotational constant which is smaller by 0.65 cm-l. These lines are almost certainly due to the first excited vibrational state of 3IZ:- NH which, from its optical spectrum [Sj, has a rotational constant Bl =Bu-0.646 cm-l. The remaining weak lines appear in the 302 b’rn spectrum, and their fourfold hyperfine patterns, illustrated by fig. lc, identify them with the isotopic molecule 15NH, in its natural abundance of approximately 0.4%.
ND, which has many coincidences with known laser lines. Because iis rotation spectrum lies beyond the reach of microwave oscillators, imine has until now been inaccessible to the precise determinations of molecular structure and of the fine-scale electric and mapetic interactions which are characteristic of spectroscopy with monochromatic sources. A systematic study of the four isotopes 14NH, 14ND, lSNH and 15ND by the laser magnetic resonance methud is no-w in progress in this laboratory. Since NH2 magnetic resonance lines can be monitored under the same conditions with NH, the optically pumped laser spectrometer should be useful for investigating reactions such as the stepwise decomposition of ammonia and hydrazine in reactions with atoms, and in flames and expiosions. An analytical instrument of this type, based on an electric discharge laser, has recently been used to study reaction rates
of OH [lo].
Referen 111 P.B. Dnvies. D.K. Russell, B.A. Thrush and F.D. Wayne, to be published. 121R.N. Dixon,Can. I. Fhys. 37 (1959) 1171. 131D.E. Millign and M.E. Jacox, J. Chem. Phys. 41 (1964) 2838. [41 K. Rosengren and G.C. Pimentel, (1965) 507. 151 H.E. Radford, IEEE I. Quantum published. [61 H.E. Radford, K.&I. Evenson and phys. 50 (1974) 3178. [71 K.M. Evenson. H.E. Radford and Phys.
4. Conclusion Much remains to be !.earned from the submillimeter spectrum of imine, in particular from the spectrum of
..
titters
J. Chem. Fhys. 43 Electron.,
to be
C-3. Howard,
J. Chem.
J.M. Moran, Appl.
18 (197 1) 426.
181 R.A. Frosch and H.M. Fo!ey, Phys. Rev. 88 (1952) 1337. [91 A-R. Edmonds, Angulv momentum in quantum me&anits (Prineton Univ. Press, Priwxton. 1960). [ial C.J. Howvd and K.M. Evenson, J. Chem. miys. 61 (1974)
1943.
564
1 August 1975
CHEMICALPHYSICSLETTERS
3
34, number
CIIEMICAL
3
1 Au~st 1975
PHYSICS LETTERS
IMINE (NH) DETECTED BY LASER MAGNETIC RESONANCE H.E. FUDFOEUI and M.M. LITVAK Center forAstrophysics. arid Smithsoriti
ffafvard College Observatory. dsfrophyskal Observatory, Cambridge,
Massachusetts
02138,
USA
Received 13 hlay 1975
The N= l+ 0 rotational spectrum of tie free NH radical in its ground ‘C- state has been meaxurcd with an opticallypumped laser mgnetic resonance spectrometer, operating at wavelengths nest 0.3 mm. The hyperfine structure constants bN = -17.3 2 1 hIHz, CN = 107.4 C 6 MHz, bH = 15.9 * 1 hiHz and cH = 150.8 f 8 MHz ue derived from the spectrum.
1. Lntroduction
The gas-phase reaction of atomic
fluorine
with
am-
mania has been found recently to generate enough amino radical (NH2) to be detectable by its laser magnetic resonance spectrum [ 11. Using an improved ver-
sion of this spectroscopic method, at wavelengths near 0.3 mm, we have now detected the imine radical (NH) in the. same reaction. Although the optical spectrum [2] of gaseous NH and the near-infrared spectrum ]3,4] OFNH frozen in solids are we!1 known and thoroughly studied, this is the fust detection of the submillimeter rotational spectrum of NH.
2. Spectrometer The spectrometer
and spectrum consists
of an optically-pumped
submillimeter gas laser, 3 inches in diameter and 6 feet long, one end of which is partitioned off by a Brewster-angle window to form an intracavity absorption cell. This section of the laser tube occupies tI;e air gap of a 1Zinch laboratory electromagnet, and it also forms the reaction vessel of a flow@g gas reaction systern. Absorption of submillimeter power by the reacting gas mixture, at field strengths that correspond to magnetic resonance, is detected by a gold-doped germznium bolomeier, cooled to 2 K, which monitors the output
power of the laser. The optical pump is a grating-tuned CO2 laser, with 3 maximum output power of 20 W.
With variouslaser gases and CO, pump lines, more than 100 cw submillimeter laser lines can be generated, at wavelengths between 40 pm and 2 mm IS]. Of
these lines about 30, thoss with pump power thresholds below 5 W, can be used in our present magnetic resonance spectrometer; pump beams of higher power puncture the polypropylene film which forms the Brewster-angle window. The pump beam enters the spectrometer through a 2-mm diameter hole in one of the end mirrors, and the same hole is used to couple out submillimeter laser power to the,cooled bolometer detector. In other respects, the present spectrometer is much like those used in the earlier work on NH, [ 11, HO, [6] and CH [7], w!dch were excited by a dc electric discharge in the laser gas. The advantage of optical excitation over electrical excitation lies in the much larger number and wider wavelength distribution of laser lines that can be generated. This is an important consideration for’a spectroscopic technique that relies on near-coincidences of laser lines with moiecular spectral lines. Strong NH, absorption spectra, at many different laser wavelengths, are observable when CF, (Freon-14j gas is passed through a microwave discharge and mixed wi’& ammonia in the absorption cell at a total
pressure of about 0.2 torr, the mixture being adjusted for the brightest green chemiluminescence. The new NH absorption spectra appear &en the ammonia flow is reduced to the point where the chemiluminescence turns orange. Since the products of an electric dis-
Volume 34; number 3
chirge in CFq are known to contain atomic fluorine, an efficient abstractor of hydrogen, the reaction path is probably hydrcgen abstraction from NH3 to form
NH,, followed by a-second hydrogen abstraction, under conditions of excess atomic fluorine, to form NH. To verify-that
a carbonaceous CF, discharge was not responsible
product of the for the new spec-
tra, SF, gas, another known source of atomic fluorine, was substituted. The same spectra appeared, although with somewhat less intensity. Representative spectral lines recorded under the onngechemiluminescence conditions are shown in fig. 1. Only two of tk thirty-odd usa‘ulc laser wave-
length&302 Drn and 3 15 Dm, gave spectra that could not @ identified with NH2 or OH. The conventional Zeeman notation TIand u is used to label lines which appear when the subznillimeter
parallel or perpendicular two
difi’erent
1 Ausst
CHEMICAL PHYSICS LETTERS
electric
field is oriented
to .Lhemagnetic field. (The
field orientations
are produced
by rota-
1975
ting the Brewster-angle window about the axis of the laser tube.) The line profiles show the usual fiistderivative shape produced by magnetic modulation and synchronous detection. Line positions were measured to an accuracy off 0.01% with 2 nuclear magnetic resonance probe, inserted cell at the center of the magnet
into the absorption gap.
The 302 urn and 315.~rn laser lines are both pumped by the 9.6 pm R(4) line of CO,, the laser gases being formic acid and methyknine, respectively. We measured the two laser frequencies to a precision of 1 part in lo5 by comparing them with harmonics of a free-running millimeter-wave klystron, the frequency of which was in turn compared with harmonics of a centimeter-wave multiplier chain. The submillimeter harmonic mixer was a gallium arsenide Schottky diode, kindly loaned for this purpose by H.R. Fetterman of the M.I.T. Lincoln Laboratory. The results
were
Y(302 flm) = 991779 5 IO hi!&, ~(315 pm) = 952185
where the uncertainty
+ 10 MHz,
is due to the frequency instab-
ility of the m’tiimeter-wave
klystron.
3. Analysis
(bl
The two laser lines kwithin the fine structure the N = i ~‘0 rotational transition of the ground
, 43i-J
I
45a5
of
(3Z-, v = 0) state of NH, as shown by the energy level diagram of fig. 2. The magnetic field dependence of tie ener,y sub-levels in this diagram was calculated as a perturbation of the zero-field energy levels by the
simple Zeeman operator
(cl. 302
d
where g, = 2.0023
is the electron spin g-factor. p. is the Bohr magneton, S is the electron spin angu!ar momentum and His the magnetic field. Only matrix. L
I
I
1554
1565
MAGNETIC FIELO STREiGM
IN GAUSS
Fig. 1. Repsentative line5 from the magnetic resonance s$xtru~ of IW, at wavelengths af 302 urn 2nd 315 Mm, in ~2rpencIiculir pohi&on. (a} 1”hH(3 C-, ,u I- 0); (t) %$pti--, P.+ !j;_(c) ‘wq3r-, Y = 0).
: ,;,.
:.,
.
elements diagonal in N were retained in the calculation. Zero-field energy levels derived from ‘Jle ultraviolet spectrum of NH were used in the drawing of fig. 2, and the same levels provided initial values of parameters fir an iterative computer fit of the spectra. Eyperfme structure was not included at this stage in the analysis; the rtiean magnetic field values of byper_..
CHEMICAL PHYSICS LETTERS
Volume 34, number 3
served, and again the two near 11 kG are not independent. With five independent data points to determine three parameters, there is enough redundancy to verify the quantum number assignments represented by fig_ 2 but hardly enough to warrant a more detailed theoretical treatment of the Zeeman effect. The values of the best-fit parameters are v(fV=1,5=2+h’0,J=1)=32.50.53
f 0.0005 cm-l
,
Av(N=l ,J=2-+1V=I ,J=O) = 0.9348 t 0.0005 cm-’
,
Av(N=1,J=1~~V=1,J=2)=0.8500~0.0005
which agree satisfactorily with the optical values shown in fig. 2. The assigned errors are estimates based on the uncertainty in the laser frequency meas-
b
i
m b
N’O
J=l
cm-1 ,
urements
and on approximations
made
in calculating
the Zeeman effect. The largest of the neglected contributions to the Zeeman effect are those of magnetic interactions between adjacent rotational levels, and the effects of orbital magnetism, each of which can shift the lines by an amount comparable to their total width of 4 MHz. The missing line of the 3 15 pm spectrum, shown dashed in fig. 2, is predicted to lie at 6827 G, in XI otherwise clear part of the spectrum. We have no convincing explanation for its absence, but it is probab!y
’
significant
that the intensities
of lines that are observed
Fy;.
agree poorly with intensities calculated for thermal equilibrium conditions in a-passive absorption cell. Further observations of the 0.3 mm spectra under different reaction conditions may resolve these intensity discrepancies. The hyperfine structure patterns of the Zeeman lines can be accounted for by computing expectation values of the magnetic dipole interaction operator,
fine structure patterns such as those of fig. 1 were taken as data points for the computer fit, together
Nhfs = (bNIN + bf.#l{) -S’+ (C&J + c&i)
I
0
m
5 MAGNETIC
FIELD
STRENGTH
I5 IN
KILOGAUSS
2. Encr,~ sublevels of the lowest two rotntiod levels of 14NH(3Z-, Y= 0) in a magnetic field. Solid YIOWSindicate the observed lines; the dashed arrow indicates an allowed transition that was not observed.
with the measured laser frequencies. TXe minimum number of parameters that is required for an adequate fit to the data is three, and we take them to be the zero-field frequency of the transition N= 1, J=2 + N= 0, J= 1, and the two fine structure separations of the zero-field hr= 1 levels. In the 302 Irm spectrum, all of the four predicted lines were observed, but the positions of the two 1 I kG lines are not independently determined by the three parmeters. Iti the 3 1.5pm spectrum, three of the faur predicted lines were ob-
-a-s
1 (2)
where k is the unit vector along the molecular axis and the subscript N refers to the nitrogen nucleus and H to the hydrogen nucleus. The coupling constants b and c are those defined by Frosch and Foley [8]- The admixed field-dependent wavefunctions necessary for computing expectation values are obtained as; a byproduct of the computer fit to,the Zeeman spectrum. The result of this czlculation, which was done by standard Spherical tensor methods [9], is an expression for the hyperfine pattern of each Zeeman line, linear ia 563
Volume 34, number
b,, CN, bx
and cH- The patterns of aUseven lines
identified by fig. 2 are found to be consistent with a single set of these i’our constants, and a least-squares fit gives the values bN = -17.3 b,
=.
5 1 MHZ,
15.9 2 1
MHz ~
cN=107.4+6MH.z,
cH = 150.8 58 MHz,
where the experimental uncertainty assigned to each fitted value is t‘hree times its standard error. This is meant to account for possible systematic error, introduced by our neglect of zlectric quadrupo!e hyperfine structure and intermediate field effects on the mag netic hyperfme structure. Both effects would cause irregdarities in the hyperfine patterns, and both were left out because the irregularities in the measured patterns were in no case greater than one half of the line width. In addition to the seven strong lines identified in fig. 2, all of which resemble fig. la, several weaker lines appeared in high-sensitivity field scans. Three lines at 3 15 pm, of which fig. 1b is representative, correspond closely to the strong 302 pm spectrum, and are consistent with ~1rotational constant which is smaller by 0.65 cm-l. These lines are almost certainly due to the first excited vibrational state of 3IZ:- NH which, from its optical spectrum [Sj, has a rotational constant Bl =Bu-0.646 cm-l. The remaining weak lines appear in the 302 b’rn spectrum, and their fourfold hyperfine patterns, illustrated by fig. lc, identify them with the isotopic molecule 15NH, in its natural abundance of approximately 0.4%.
ND, which has many coincidences with known laser lines. Because iis rotation spectrum lies beyond the reach of microwave oscillators, imine has until now been inaccessible to the precise determinations of molecular structure and of the fine-scale electric and mapetic interactions which are characteristic of spectroscopy with monochromatic sources. A systematic study of the four isotopes 14NH, 14ND, lSNH and 15ND by the laser magnetic resonance methud is no-w in progress in this laboratory. Since NH2 magnetic resonance lines can be monitored under the same conditions with NH, the optically pumped laser spectrometer should be useful for investigating reactions such as the stepwise decomposition of ammonia and hydrazine in reactions with atoms, and in flames and expiosions. An analytical instrument of this type, based on an electric discharge laser, has recently been used to study reaction rates
of OH [lo].
Referen 111 P.B. Dnvies. D.K. Russell, B.A. Thrush and F.D. Wayne, to be published. 121R.N. Dixon,Can. I. Fhys. 37 (1959) 1171. 131D.E. Millign and M.E. Jacox, J. Chem. Phys. 41 (1964) 2838. [41 K. Rosengren and G.C. Pimentel, (1965) 507. 151 H.E. Radford, IEEE I. Quantum published. [61 H.E. Radford, K.&I. Evenson and phys. 50 (1974) 3178. [71 K.M. Evenson. H.E. Radford and Phys.
4. Conclusion Much remains to be !.earned from the submillimeter spectrum of imine, in particular from the spectrum of
..
titters
J. Chem. Fhys. 43 Electron.,
to be
C-3. Howard,
J. Chem.
J.M. Moran, Appl.
18 (197 1) 426.
181 R.A. Frosch and H.M. Fo!ey, Phys. Rev. 88 (1952) 1337. [91 A-R. Edmonds, Angulv momentum in quantum me&anits (Prineton Univ. Press, Priwxton. 1960). [ial C.J. Howvd and K.M. Evenson, J. Chem. miys. 61 (1974)
1943.
564
1 August 1975
CHEMICALPHYSICSLETTERS
3