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Raman absorption spectroscopy of crystals
Volume 63. number 3
15 May 1979
CHEMICAL PIIYSICS LIs-rEERs
RAMAN .ABSORPTION SPECTKOSCOPY OF CRYSTALS PhiIip A. FREEDSIAX and W_Jeremy JONES 77~ EdwucI Davies Citemical Laborarories. UtlirVersiryColiexe of Skies, -4be
UK
Receiwxi IS December I978
A nersty derctoprd cq?erimmrf method for recording Raman absorption spcctr3 is described and its potential in sohd and ps stud& is discused. The AIU_ R~nun mode in ~~fcktlA IO86 cm-l recorded b\-this technique is presented 3s 3” ii-
Iustrtttron.
it has Ions been clear that the esperimentd
Iiniita-
tions of convention,tl Ranan spectroscopy wou!d prevent the study of the spectra of gases and soolids at very high resolution_ To overcome this b3rrier new methods of recording Ramzm spzctrrr h3ve been sought; the repkement of the spectrometer or spectrograph with J tunable dye Izer ;IS anaIyser, hence removing the resolution restrictions of such instruments. is J most desirable design criterion_ One of the first reason3bly successful approaches was the deveiopment of coherent anti-Stokes Ran1.m spectroscopy (CARS) as an alternative to conventional studies [I, ?I, although Iineshape problems and a quadratic signal/concentntion relationship has resuited in the technique not having the gene4 appIicabiIity first envisaged_ Because of these diffkdties we decided to try to develop 3 different technique brtsed on the inverse Rrim;ln process [3] _In the original esperiment in which Ranan absorption spectra were observed, ;t high power 10 ns pulse of laser radiation at 6943 ntn K;)S mived with continuum radiation in ;I liquid sample- Absorption from this continuum ws found at ;I wavenumber displacement from 694.3 nm corresponding ro J Raman-active transition in the sample- Our present experiments using low-power continuqus wave in place of pulsed lasers were started in 1976. Because of the low power levels employed, with a resultant low frnctionaI absorption, it 113s taken some considerabre time to eliminate the v3rious noise components which m3ssk the absorption signals_ Owyoung 2nd co-workers [4,5] recentIy re378
ported spectra in liquids and g.~ses, which they term stimulated Ram.m spectra, using 3n spprortch which is very similar to the one we have adopted_ In this note we shal1 describe our technique and stress one further aspect of the process_ The basic idea behind the Raman absorption process may be easily visuaiised by considering the technique 3s 3 simpIe two-photon experiment_ In the “more famiIiar” tao-photon experiment two incident photons are destroyed in promoting 3 molecule from the ground srxte to 3n excited level where the sum of the energy of the two photons is equal to the mo-
Iecular energy gap. In the experiment
under consideration here, a photon is created (under the stimuhting influence of radiation of the same frequency) w!liIe another of higher frequency is destroyed when the frequency ciifference corresponds to an energy gap in the molecuIe. Just 3s DoppIer broadening in the phcton-sum experiment can be greatly reduced by having the two interacting photons trrrvelling in opposite directions, in the Raman analogue narrowing occurs \\hen the two beams are propagated together_ The resolution Iimitation of the technique for g3ses is thus very high and can be JS low as 3 Few megahertz, primarily being limited by the performance of the lasers. Since this is 3 two-photon effect and we 3re using photon wwenumber differences of == SO--3000 cm-’ this is 3 powerftd 3Itemative to conventionaI Ramltn studies. It can be used to great advanktge not only in g3s-phase studies but 3Iso in the study of solids, especkdly at low temperatures where the lines
Volume
63, number 2
CHEWCAL
PHYSICS
15 >Iay
LETTERS
1979
He_Ne ’ LASER 0 Fig.
may become describe
extremely
narrow_
our experimental
1. Optical armngemcnt In this paper
usrd m R~rmn absorption
we shall
and show a of calcite to dlustrste
arrangement
room-temperature spectrum the power of this approach. The experimental arrangement is s!lo\Qn in fig. 1. The dye laser is an Edinburgh Instruments DL300 capable of a line width of= 0.05 cm-l and pumped
with a home-built
argon-ion
laser (recently
rephced
with a Coherent Radiation CRS laser). In the esperiment described the line width is = 0.5 cm-I with = 25 mW at the sample. i\fter passing through some bezmwuatching optics it passes through a Pockels cell operated at 10 k1 Ir and combines \cith radiation from an unmodulated helium-neon laser on the beam spIitter as S~OWII. This heiium-neon laser is either rl Tropel single-frequency unit (0.5 m\\‘) or dn
measurcmrnts.
mzlximum biretiingence ([I IO] in reMion to the rhomb axes) is parallel to the polzlrlzrttion direcrlon of the helium-neon reference laser. The combinrttion of the birefringence of the crystai together with the polarization change (honrontal to vertical) induced in the dye-laser radiation by the Pekels cell produces sn amplitude modulation of the pump beam st the focus of the probe beam in the cryst.d. Interaction of the 63X3 wn probe beam with Z=593-l nm rJdiation results in the spectrum shown in fig. 7 as the dye laser 1~avelen$h is scamed. The time constant w.ls 30 5.
oven-stabilised 2 mN’ Coherent Radiation CR-SO-2 ix.er_ Noise due to feed back along the dye laser path of scattered helium-neon light is minimised bq modubting the folding mirror at 1 kI_Iz 3s shown in fig 1 _ After being focused through the sampIe and re-co& limated, the two beams are separated by use of 3 double grating arrangement and the 633 nm radiAtion focused onto the surface of ZIsilicon PIK diode. To minimise further the noise from the helhun-neon laser 10% of the beam is focused onto 3 similar diode before combination with the dye laser radiation, and the two dc levels balanced and subtmcted. In the experiment with the sohd sample, .I -I mm thick crystal of calcite is placed at the focus of the two coincident laser beams. The crystal firis been polished on its cleavqe planes, normal to the incident laser radiation. and oriented so that the direction of
Fig_ 1. The XI~ mode of rhe CO:-
ion in calcite .xt IOS6 cm-‘.
379
Volume 63. number 3
CHEMICAL PHYSICS LETTERS
The spectrum of the A,, totally symmetric breathing mode of the CO% ion % calcite shown in fig_ 2 does not represent our uItimate sensitivity. Apart from gains in signal-to-noise to be obtained from the reduction of transmission losses in the optical path (thereby increasing the helium-neon signal at the detector and dye laser intensity at the sample), a significant improvement is possible by the incorpor~tion of a multipass cell into the appsmtus. However, the most important gain wiil undoubtedly arise from the line narrowing as the sample is cooIed to cryogenic temperatures, where most interest lies_ Park [G] has shown the dramatic effect on the Iine width of this AEg mode of caicite of Iowering the sample temperature from 160 K to 53 K_ The limiting resolution of this study using conventional techniques ~3s 0.4 cm-l , compar3bIe to the resolution attained in the present work_ What is not readily apparent is that in a conventional study using a spectrograph or spectrometer for a&sing scattered Raman radiation the sign&to-noise ratio diminishes as the resoIution is improved for tile investigation of very narrow lines whereas in the Raman absorption process the signdto-noise ratio increases_ With 3 limiting resoiution of z 0.05 cm-l for our equipment at the present time, the adv3nr3ges for the investigation of very narrow Line spectra in cryslais at low temperatures are obvious. In addition, the use of a we11 defined interaction direction can aIso provide vaIuabIe pokwizabili-
380
15 May 1979
ty information difficult to obtain from conventional Raman scattering studies in the forward direction_ Furfher studies on c3Icite and on other soIid 3nd g3seous samples are continuing_ We should like to acknowIedge very helpfu1 discussions on this problem over 3 number of years with Professor B.P. Stoicheff- We are indebted to the Science Research Council for the award of 3 research grant in 1976, with the aid of wNch the equipment employed in this work was developed_ A further gr3nt awarded in 1978 wiII be of great value in extending our studies in this area_ It is also 3 pleasure to thsnk Trinity CoI!ege, GInbridge, for the award of a Title A Fellowship to P-A-F.
References [ 1 I P-D. Maker and R-U’_Terhunr, Phys_ Rev_ 137
(1965)
A601. [2)
3-L
Barrett and R-F. Begley, Appl. Phvs. Letters 27
(1975) 129. ISI 1Y-J.Jonesand B.P. Stoicheff, Ph_vs_Rev- Letters 13 (1964) 657_ [?I A. Owyoung. Opr- Commun. 22 (1977) 373. [S 1 A_ Oxxyouns. C-W_ Patterson and R.S. McDor~elI, Chem. Phys_ J_ettcrs 59 (1978) 156. [61 k. I’.uk, Phys. Letters 21 (1966) 39;25A (1967) 490_
15 May 1979
CHEMICAL PIIYSICS LIs-rEERs
RAMAN .ABSORPTION SPECTKOSCOPY OF CRYSTALS PhiIip A. FREEDSIAX and W_Jeremy JONES 77~ EdwucI Davies Citemical Laborarories. UtlirVersiryColiexe of Skies, -4be
UK
Receiwxi IS December I978
A nersty derctoprd cq?erimmrf method for recording Raman absorption spcctr3 is described and its potential in sohd and ps stud& is discused. The AIU_ R~nun mode in ~~fcktlA IO86 cm-l recorded b\-this technique is presented 3s 3” ii-
Iustrtttron.
it has Ions been clear that the esperimentd
Iiniita-
tions of convention,tl Ranan spectroscopy wou!d prevent the study of the spectra of gases and soolids at very high resolution_ To overcome this b3rrier new methods of recording Ramzm spzctrrr h3ve been sought; the repkement of the spectrometer or spectrograph with J tunable dye Izer ;IS anaIyser, hence removing the resolution restrictions of such instruments. is J most desirable design criterion_ One of the first reason3bly successful approaches was the deveiopment of coherent anti-Stokes Ran1.m spectroscopy (CARS) as an alternative to conventional studies [I, ?I, although Iineshape problems and a quadratic signal/concentntion relationship has resuited in the technique not having the gene4 appIicabiIity first envisaged_ Because of these diffkdties we decided to try to develop 3 different technique brtsed on the inverse Rrim;ln process [3] _In the original esperiment in which Ranan absorption spectra were observed, ;t high power 10 ns pulse of laser radiation at 6943 ntn K;)S mived with continuum radiation in ;I liquid sample- Absorption from this continuum ws found at ;I wavenumber displacement from 694.3 nm corresponding ro J Raman-active transition in the sample- Our present experiments using low-power continuqus wave in place of pulsed lasers were started in 1976. Because of the low power levels employed, with a resultant low frnctionaI absorption, it 113s taken some considerabre time to eliminate the v3rious noise components which m3ssk the absorption signals_ Owyoung 2nd co-workers [4,5] recentIy re378
ported spectra in liquids and g.~ses, which they term stimulated Ram.m spectra, using 3n spprortch which is very similar to the one we have adopted_ In this note we shal1 describe our technique and stress one further aspect of the process_ The basic idea behind the Raman absorption process may be easily visuaiised by considering the technique 3s 3 simpIe two-photon experiment_ In the “more famiIiar” tao-photon experiment two incident photons are destroyed in promoting 3 molecule from the ground srxte to 3n excited level where the sum of the energy of the two photons is equal to the mo-
Iecular energy gap. In the experiment
under consideration here, a photon is created (under the stimuhting influence of radiation of the same frequency) w!liIe another of higher frequency is destroyed when the frequency ciifference corresponds to an energy gap in the molecuIe. Just 3s DoppIer broadening in the phcton-sum experiment can be greatly reduced by having the two interacting photons trrrvelling in opposite directions, in the Raman analogue narrowing occurs \\hen the two beams are propagated together_ The resolution Iimitation of the technique for g3ses is thus very high and can be JS low as 3 Few megahertz, primarily being limited by the performance of the lasers. Since this is 3 two-photon effect and we 3re using photon wwenumber differences of == SO--3000 cm-’ this is 3 powerftd 3Itemative to conventionaI Ramltn studies. It can be used to great advanktge not only in g3s-phase studies but 3Iso in the study of solids, especkdly at low temperatures where the lines
Volume
63, number 2
CHEWCAL
PHYSICS
15 >Iay
LETTERS
1979
He_Ne ’ LASER 0 Fig.
may become describe
extremely
narrow_
our experimental
1. Optical armngemcnt In this paper
usrd m R~rmn absorption
we shall
and show a of calcite to dlustrste
arrangement
room-temperature spectrum the power of this approach. The experimental arrangement is s!lo\Qn in fig. 1. The dye laser is an Edinburgh Instruments DL300 capable of a line width of= 0.05 cm-l and pumped
with a home-built
argon-ion
laser (recently
rephced
with a Coherent Radiation CRS laser). In the esperiment described the line width is = 0.5 cm-I with = 25 mW at the sample. i\fter passing through some bezmwuatching optics it passes through a Pockels cell operated at 10 k1 Ir and combines \cith radiation from an unmodulated helium-neon laser on the beam spIitter as S~OWII. This heiium-neon laser is either rl Tropel single-frequency unit (0.5 m\\‘) or dn
measurcmrnts.
mzlximum biretiingence ([I IO] in reMion to the rhomb axes) is parallel to the polzlrlzrttion direcrlon of the helium-neon reference laser. The combinrttion of the birefringence of the crystai together with the polarization change (honrontal to vertical) induced in the dye-laser radiation by the Pekels cell produces sn amplitude modulation of the pump beam st the focus of the probe beam in the cryst.d. Interaction of the 63X3 wn probe beam with Z=593-l nm rJdiation results in the spectrum shown in fig. 7 as the dye laser 1~avelen$h is scamed. The time constant w.ls 30 5.
oven-stabilised 2 mN’ Coherent Radiation CR-SO-2 ix.er_ Noise due to feed back along the dye laser path of scattered helium-neon light is minimised bq modubting the folding mirror at 1 kI_Iz 3s shown in fig 1 _ After being focused through the sampIe and re-co& limated, the two beams are separated by use of 3 double grating arrangement and the 633 nm radiAtion focused onto the surface of ZIsilicon PIK diode. To minimise further the noise from the helhun-neon laser 10% of the beam is focused onto 3 similar diode before combination with the dye laser radiation, and the two dc levels balanced and subtmcted. In the experiment with the sohd sample, .I -I mm thick crystal of calcite is placed at the focus of the two coincident laser beams. The crystal firis been polished on its cleavqe planes, normal to the incident laser radiation. and oriented so that the direction of
Fig_ 1. The XI~ mode of rhe CO:-
ion in calcite .xt IOS6 cm-‘.
379
Volume 63. number 3
CHEMICAL PHYSICS LETTERS
The spectrum of the A,, totally symmetric breathing mode of the CO% ion % calcite shown in fig_ 2 does not represent our uItimate sensitivity. Apart from gains in signal-to-noise to be obtained from the reduction of transmission losses in the optical path (thereby increasing the helium-neon signal at the detector and dye laser intensity at the sample), a significant improvement is possible by the incorpor~tion of a multipass cell into the appsmtus. However, the most important gain wiil undoubtedly arise from the line narrowing as the sample is cooIed to cryogenic temperatures, where most interest lies_ Park [G] has shown the dramatic effect on the Iine width of this AEg mode of caicite of Iowering the sample temperature from 160 K to 53 K_ The limiting resolution of this study using conventional techniques ~3s 0.4 cm-l , compar3bIe to the resolution attained in the present work_ What is not readily apparent is that in a conventional study using a spectrograph or spectrometer for a&sing scattered Raman radiation the sign&to-noise ratio diminishes as the resoIution is improved for tile investigation of very narrow lines whereas in the Raman absorption process the signdto-noise ratio increases_ With 3 limiting resoiution of z 0.05 cm-l for our equipment at the present time, the adv3nr3ges for the investigation of very narrow Line spectra in cryslais at low temperatures are obvious. In addition, the use of a we11 defined interaction direction can aIso provide vaIuabIe pokwizabili-
380
15 May 1979
ty information difficult to obtain from conventional Raman scattering studies in the forward direction_ Furfher studies on c3Icite and on other soIid 3nd g3seous samples are continuing_ We should like to acknowIedge very helpfu1 discussions on this problem over 3 number of years with Professor B.P. Stoicheff- We are indebted to the Science Research Council for the award of 3 research grant in 1976, with the aid of wNch the equipment employed in this work was developed_ A further gr3nt awarded in 1978 wiII be of great value in extending our studies in this area_ It is also 3 pleasure to thsnk Trinity CoI!ege, GInbridge, for the award of a Title A Fellowship to P-A-F.
References [ 1 I P-D. Maker and R-U’_Terhunr, Phys_ Rev_ 137
(1965)
A601. [2)
3-L
Barrett and R-F. Begley, Appl. Phvs. Letters 27
(1975) 129. ISI 1Y-J.Jonesand B.P. Stoicheff, Ph_vs_Rev- Letters 13 (1964) 657_ [?I A. Owyoung. Opr- Commun. 22 (1977) 373. [S 1 A_ Oxxyouns. C-W_ Patterson and R.S. McDor~elI, Chem. Phys_ J_ettcrs 59 (1978) 156. [61 k. I’.uk, Phys. Letters 21 (1966) 39;25A (1967) 490_