- Email: [email protected]
Measurement of the pressure broadening of the rotational raman lines of HCl
.~olumell,number’3
,:
C~EMICk.
..,.‘,
PHYd
&ETTERi
:.
.. .,
‘. ,_‘. .’
.-
., .
.
:
.’
.,
. .
.,
‘..’
._
‘,..
. : .. .’
:.. 1~
. .
‘:
MEASUREMENT’OF dF T~,R~TA~~NAL
NT& RkH and H:L:WEI..SH Department ofPhysics, Unhwsfty of T&onto, Torixzto, Ontark2~ Canada
I : ‘.I,
.’ :
THE PRESSURE BROADENING RA1MAIL’LINES OF ffCl* -
:
Received ~~3~~~1971
;: Experimentat &f-broadening coefficients of the HCi rotational Raman lines show a maximum atJow J followed by a rapid decrease with inawsiq J, in accord with recent Qlcuktions by Gray. However, the theoretical vaiues are on the average .&60% higher.
,,
.,
‘.
,A ieview of experimental and. theoretical studies of the pressure broadening of Haman lines has been given recently by Gray and Welsh [l] .’Defmite experimen-
tal results on rotational Paman spectra have been ob&ed &my foi the nonpolar &e’s Hz [2], O,, N,, _ andXX&, and the weakIy polar gas CO 133 . Since the theory [4] shows that the broadening of rotational, H+man lines of SrongIy dipolar gases should show marked differences from nonpolar gases, we present here some initial experimental results on hydrogen chIcwide.
The gas was contained in a cylindrical glass cell, 1.1 cm in diameter. and 8 cm long; a plane window fused to one end of the cell admitied the focused beam from a CRL Model 52 argon ion laser. The 4880 A line bf argon, for which the beam power was about 1 W, was used as the exciting.iine. Light scattered . through the ,verti&l walls of $e cefl was collected by a,lens sjstem with a sphericalmirror behind the cell to incr?ase the collection efficiency. Since the rotational Raman lines are depolarized a,Polaroid film was used. to discriminate against the intense polarized exciting line.,The spectra w&e recoided in the second “order of a Spex sc+ning double mon~~~o~ator with 1200 lines/mirn gratings blazed at 7500 & the detector was an TM’&%130 photom~I~p~er tube, followed by I ‘.**
:. .: ., rkarcb WBS sup&&i in part i$ ihg.‘NatimrRe-
sear&Cotidlof,Canapa;
.:’
- ‘.
.’
:.’
pulse counting electronic equipment. The spectral slit widths were 1.5 cm-l. Measurements of the widths of the HCI Stokes rotational lines were made for 8 pressures in the range 3 to I4 atm at room temperature. The frequency scale was established from the known spacings of the rota-
tion lines. Over the pressure range used the iinewidths were found to vary linearly with pressure, and broadening coefficients were found from a least squares line;u fit of the data from two different runs. The self-broadening coefficients, in the form Au/p
where Aa is the half width (cm-l) at one-half the peak intensity and p is the pressure (atm), are plotted in fig. 1 .as a function of the rotational quantum number J; for comparison,, experimental data for nitrogen from ref. [3] ari: also given. Severaf features of the J dependence of the broadening coefficients for NCI are in distinct contrast with the results for the typicai nonpolar molecule, Nz: (i) for loy,J the mag~tude is = 3 times greater, (ii) a peak occurs at low,J, and (iii) the coeffiiient decreases more rapidly wi*J increasing J. ” The b~oade~g ~~f~~ie~ts calculated by Gray [4] for, HC! are also *own in fq. 1; The peak at low J and, the rapid decrdase with in&easing 3 are,well reproduced by the’.tieory..However, on th< average the theoretical
v@uesare,k: $074higher tl+ the’experiqenti values. On&aspect of tðemetic results which is kft un.. . ..’
.
;’ ,. _:.
‘.’ ._
,‘.
,’ :. _;... .I
.:;
...
‘.
-:.
Volume 11, number 3
CHEMICAL PHYSICS LETTERS
a l
0
0
0
l
l
0
0
l
**AAAA
AA
l l
A
A
A
J
Fig. 1. Pressure-broadening Raman lines as a function
coefficients (Au/p) of rotational of J: HCI - experimental (open
circles), HCI - theoretical [4] (filled circles). N2 - qxperimental [ 31 (triangles).
confirmed from these experiments is the predicted oscillation of the broadening coefficient at large values of J. The rotational scattering of HCl is extremely weak, more than an order of magnitude weaker than
‘1.5 October 1971
cal reactivity has prevented us from using successfully a multiple-reflection mirror system [S] _Thus, the signal-to-noise ratio is poor for the weaker lines at high J values (J> 8) where the alternation is expected to occur. In conclusion, we remark that improvement of these results will require not onIy &creasing the signalto-noise ratio, but also correc;ing for the apparent broadening caused by the superpbsition of transitions of H% and H37C1.The frequency separation of the lines for the isotopic moIecules increases withJ, so that the effect is largest in the region where the interesting alternation effect is expected.
References [l] C.G. Gray and H.L. Welsh, in: Essays 5~ structural
chem-
istry, eds. DA: Long, AJ. Downsand LA.K. Stavely (Macmillan, London, 1971). [2] V.G. Cooper, A.D. May, E.H. Hara and H.F.P. Knaap, can. J. Phys. 46 (1968) 2019. [3] KS. Jammu. G.E. St. John and H.L. Welsh, ban. I. Phys. 44 (1966) 797. [4] C.G. Gray, Chem. Phys. Letters 8 (i971) 528. l51 N.H. Rich and H.L.Welrh, J-Opt. .%c Am. 61(1971)
that of .a simple homopolar moIecule, and its chbmi-
293
,:
C~EMICk.
..,.‘,
PHYd
&ETTERi
:.
.. .,
‘. ,_‘. .’
.-
., .
.
:
.’
.,
. .
.,
‘..’
._
‘,..
. : .. .’
:.. 1~
. .
‘:
MEASUREMENT’OF dF T~,R~TA~~NAL
NT& RkH and H:L:WEI..SH Department ofPhysics, Unhwsfty of T&onto, Torixzto, Ontark2~ Canada
I : ‘.I,
.’ :
THE PRESSURE BROADENING RA1MAIL’LINES OF ffCl* -
:
Received ~~3~~~1971
;: Experimentat &f-broadening coefficients of the HCi rotational Raman lines show a maximum atJow J followed by a rapid decrease with inawsiq J, in accord with recent Qlcuktions by Gray. However, the theoretical vaiues are on the average .&60% higher.
,,
.,
‘.
,A ieview of experimental and. theoretical studies of the pressure broadening of Haman lines has been given recently by Gray and Welsh [l] .’Defmite experimen-
tal results on rotational Paman spectra have been ob&ed &my foi the nonpolar &e’s Hz [2], O,, N,, _ andXX&, and the weakIy polar gas CO 133 . Since the theory [4] shows that the broadening of rotational, H+man lines of SrongIy dipolar gases should show marked differences from nonpolar gases, we present here some initial experimental results on hydrogen chIcwide.
The gas was contained in a cylindrical glass cell, 1.1 cm in diameter. and 8 cm long; a plane window fused to one end of the cell admitied the focused beam from a CRL Model 52 argon ion laser. The 4880 A line bf argon, for which the beam power was about 1 W, was used as the exciting.iine. Light scattered . through the ,verti&l walls of $e cefl was collected by a,lens sjstem with a sphericalmirror behind the cell to incr?ase the collection efficiency. Since the rotational Raman lines are depolarized a,Polaroid film was used. to discriminate against the intense polarized exciting line.,The spectra w&e recoided in the second “order of a Spex sc+ning double mon~~~o~ator with 1200 lines/mirn gratings blazed at 7500 & the detector was an TM’&%130 photom~I~p~er tube, followed by I ‘.**
:. .: ., rkarcb WBS sup&&i in part i$ ihg.‘NatimrRe-
sear&Cotidlof,Canapa;
.:’
- ‘.
.’
:.’
pulse counting electronic equipment. The spectral slit widths were 1.5 cm-l. Measurements of the widths of the HCI Stokes rotational lines were made for 8 pressures in the range 3 to I4 atm at room temperature. The frequency scale was established from the known spacings of the rota-
tion lines. Over the pressure range used the iinewidths were found to vary linearly with pressure, and broadening coefficients were found from a least squares line;u fit of the data from two different runs. The self-broadening coefficients, in the form Au/p
where Aa is the half width (cm-l) at one-half the peak intensity and p is the pressure (atm), are plotted in fig. 1 .as a function of the rotational quantum number J; for comparison,, experimental data for nitrogen from ref. [3] ari: also given. Severaf features of the J dependence of the broadening coefficients for NCI are in distinct contrast with the results for the typicai nonpolar molecule, Nz: (i) for loy,J the mag~tude is = 3 times greater, (ii) a peak occurs at low,J, and (iii) the coeffiiient decreases more rapidly wi*J increasing J. ” The b~oade~g ~~f~~ie~ts calculated by Gray [4] for, HC! are also *own in fq. 1; The peak at low J and, the rapid decrdase with in&easing 3 are,well reproduced by the’.tieory..However, on th< average the theoretical
v@uesare,k: $074higher tl+ the’experiqenti values. On&aspect of tðemetic results which is kft un.. . ..’
.
;’ ,. _:.
‘.’ ._
,‘.
,’ :. _;... .I
.:;
...
‘.
-:.
Volume 11, number 3
CHEMICAL PHYSICS LETTERS
a l
0
0
0
l
l
0
0
l
**AAAA
AA
l l
A
A
A
J
Fig. 1. Pressure-broadening Raman lines as a function
coefficients (Au/p) of rotational of J: HCI - experimental (open
circles), HCI - theoretical [4] (filled circles). N2 - qxperimental [ 31 (triangles).
confirmed from these experiments is the predicted oscillation of the broadening coefficient at large values of J. The rotational scattering of HCl is extremely weak, more than an order of magnitude weaker than
‘1.5 October 1971
cal reactivity has prevented us from using successfully a multiple-reflection mirror system [S] _Thus, the signal-to-noise ratio is poor for the weaker lines at high J values (J> 8) where the alternation is expected to occur. In conclusion, we remark that improvement of these results will require not onIy &creasing the signalto-noise ratio, but also correc;ing for the apparent broadening caused by the superpbsition of transitions of H% and H37C1.The frequency separation of the lines for the isotopic moIecules increases withJ, so that the effect is largest in the region where the interesting alternation effect is expected.
References [l] C.G. Gray and H.L. Welsh, in: Essays 5~ structural
chem-
istry, eds. DA: Long, AJ. Downsand LA.K. Stavely (Macmillan, London, 1971). [2] V.G. Cooper, A.D. May, E.H. Hara and H.F.P. Knaap, can. J. Phys. 46 (1968) 2019. [3] KS. Jammu. G.E. St. John and H.L. Welsh, ban. I. Phys. 44 (1966) 797. [4] C.G. Gray, Chem. Phys. Letters 8 (i971) 528. l51 N.H. Rich and H.L.Welrh, J-Opt. .%c Am. 61(1971)
that of .a simple homopolar moIecule, and its chbmi-
293