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Laser-induced gratings in oxygen excited via the b 1Σg+(v′ = 0) state
21 June 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 256 (1996) 71-76
Laser-induced gratings in oxygen excited via the b l g(V' = 0) state B. Hemmerling, R. Bombach, W. Hubschmid Paul Scherrer lnstitut, CH-5232 Villigen PSI, Switzerland Received 28 November 1995; in f'mal form 12 March 1996
Abstract
Laser-induced gratings have been generated by exciting transitions in the red system of molecular oxygen. The temporal evolution of the grating reflectivity has been studied at room temperature and atmospheric pressure. Conlributions to the grating reflectivity arise from a change in population distribution caused by absorption, from electrostriction, and from a release of absorbed laser energy in form of heat. Adding some water to the oxygen sample strongly increases the grating reflectivity rendering the laser-induced grating method sensitive enough to detect the isotopic molecule 160180 in its natural abundance.
I. Introduction
Laser-induced gratings (LIGs) are formed in a medium by various resonant and nonresonant mechanisms as response on the spatially sinusoidal intensity pattern generated by the interference of two intersecting laser beams. Some of the mechanisms have been investigated in detail: electrostrictive gratings are generated at any frequency of the excitation beams and have been observed in time resolved transient grating studies in solids and liquids [1] and in gases [2-5]. In molecules or atoms resonant with the frequency of the incident laser light, a spatially modulated population difference between excited and ground state is generated. Such population gratings are dealt with in degenerate four-wave mixing experiments [6]. Eventually, the excited state energy is released as heat leading to thermal gratings [7,8].
Their potential use for diagnostics in flames has been shown in Ref. [9]. The variety of different contributions to the grating reflectivity may render the interpretation of a measured LIG signal difficult. However, there is the possibility to tailor the experiment to select and enhance one of the contributions to the signal in order to allow for a more sensitive species detection. Furthermore, some of the contributions to the grating reflectivity are based on molecular energy transfer. Analysis of the temporal development of the LIG signal may therefore give some insight into molecular kinetics. In order to analyse the mechanisms contributing to LIGs the gratings are generated by exciting lines in the b 1 ~ - ~ X 3~g system of oxygen. The metastable character of the singlet state allows the investigation of slow nonradiative processes. Fur-
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All fights reserved PH S0009-261 4(96)0041 2-5
72
B. Hemmerling et aL / Chemical Physics Letters 256 (1996) 71-76
thermore, energy transfer processes involving singlet states of 0 2 are of great and continuous interest in atmospheric chemistry and biology. Therefore, on the other hand, LIGs may prove to be a useful method to study molecular energy transfer processes.
trum with a path length of 10 m in a cell containing pure oxygen at 1 bar was taken. In order to allow for time-resolved acquisition of single pulse signals, a digitizer (Tektronix TDS 544A) with a bandwidth of 500 MHz and a sampling rate of 109 s - l was employed.
2. Experimental A scheme of the experimental apparatus is shown in Fig. 1. The two grating excitation beams were provided by a single-longitudinal-mode optical parametric oscillator (Continuum HRL-100Z). The pulse length was about 5 ns and the bandwidth is specified to be smaller than 500 MHz. Focused by a lens ( f = 1000 mm), the two excitation beams overlapped each other at the focus which was located in a sample ceil. At this point the total intensity was approximately 10 G W / c m 2. The angle 0 between the two excitation beams was about 1°. A second lens ( f = 1000 mm) was used to focus the counterpropagating beam of a cw Ar + laser (Innova 70-4) reading out the laser-induced grating in a planar backward phase matching geometry. The power of the Ar ÷ laser was 1.7 W running in single line mode at A = 514.5 nm. Oxygen with a purity of 99.5% was used throughout the measurements and no special care was taken to remove residual water from the gas or the sample cell. Synchronously to the recording of the laser-induced grating spectrum, the laser power was monitored and an absorption spec-
I ~
_
.
BS2
_
BS3 ~
t
~
• :
Ar ÷
I
T I I
Fig. 1. Experimental apparatus. BSI, beamsplitter ( R = 10%); BS2 and BS3, beamsplitter (R = 50%); FP, Fabry-Perot interferometer; WM, wave meter; PM, photomultiplier tube; D, digitizer.
3. Results and discussion The diffraction efficiency of a laser-induced grating is, see e.g. Ref. [10], =
+
(1)
where d is the thickness of the grating, A is the wavelength of the excitation laser, and A n and A K are the change of the refractive index and of the absorption coefficient across a fringe of the grating, respectively. If there is no molecular/atomar resonance at the wavelength of the laser reading out the grating, A K is equal to zero and a pure phase grating is obtained. The refractive index modulation can phenomenologically be described by
An= ~ij ANij+ -~ oAT+t-~-fi]r p
(2)
where N/; is the deviation from the thermal population difference between the two molecular levels i and j connected by the frequency of the excitation laser. The first term in Eq. (2) accounts for the influence of the population transfer caused by the excitation laser on the refractive index at the wavelength of the read-out laser. Once the absorbed energy is thermalized the refractive index is a function of (local) thermodynamic quantities. In a gas the thermo-optic coefficient (3n/ST) is usually small and can be neglected in comparison to the change of the refractive index caused by the variation in density. The density change effected by electrostriction and by thermalization of absorbed laser energy can be calculated by solving the linearized fluid dynamic equations, see e.g. Ref. [11]. The solution of these equations is a stationary density wave at constant pressure superimposed on a standing acoustic wave
B. Hemmerling et al./ Chemical Physics Letters 256 (1996) 71-76
with contributions arising from electrostriction and heat release. Viscosity and heat conduction damp the acoustic wave. The stationary wave is damped by heat conduction alone. The oscillation period of the grating reflectivity is Tg = h / [ 2 v sin(0/2)] where v is the adiabatic sound velocity. If no internal energy is released in form of heat, the amplitude of the stationary density wave is much smaller than the one of the density change of the acoustic wave. In this case of pure electrostriction, the density oscillates around its undisturbed value leading to an oscillation period of the grating reflectivity of Tg/2. The properties of LIGs formed by thermalization of absorbed laser energy depend on the time constant T~ of the energy release. Fast energy release (Tg >> 2"rrTr) generates a sound wave and a stationary density wave with equal amplitude. Slow energy release (Tg << 2~rT~) favours the generation of the stationary density wave whereas the development of sound waves is hindered due to the destructive superposition of sound waves generated at different times. Fig. 2 shows the temporal behaviour of the reflectivity for a grating generated by a single pulse of the excitation laser in a mixture of 1 bar oxygen and 0.05 bar carbon dioxide. The wavelength of the laser was tuned to the PP(9) line in the 0 - 0 band of the b lX~- ,_ X 3Xg system of oxygen. The appearance of the oscillating part of the signal resembles the one of a signal resulting from a superposition of a thermal and an electrostrictive grating. After the oscillating part has vanished, due to damping and because the sound waves have left the interaction volume, a broad unstructured hump appears. In a mixture of 1 bar oxygen and 0.05 bar carbon dioxide the time constant for quenching of O 2 in the b 1Eg+ ( v ' = 0) state is 3.5 I~s [12]. This is too slow to excite an acoustic amplitude from the thermalization of absorbed laser energy which is comparable to the one excited by electrostriction. A calculation shows that for slow thermalization the reflectivity of the oscillating part of the grating is reduced by a factor of the o r d e r TgTd/4"rrTr 2 ( T d is the dissipation time of the grating) compared to fast thermalization with the same energy release. Therefore, the small amount of internal energy converted t o / f r o m translation energy which accompanies rotational state redistribution within the ensemble of molecules involved in the excitation process can generate a stronger
73
1.0
c
z.
m
0.5
c
0.0 i
t
i
I
0.0
5.0
10.0
15.C
time / tJs 1.0
b e-
o.s
g 0.0 0.o
o.s
1.0
1.s
time / /JS
Fig. 2. (a) Single-pulse LIG signal obtained in a mixture of 1 bar 02 and 0.05 bar CO 2. (b) The first part of the signal with higher temporal resolution.
oscillating grating than the energy release which results from subsequently following slow energy transfer processes. On the other hand, the energy release by electronic quenching is too fast to be responsible for the broad unstructured hump visible in Fig. 2b at about 5 ixs. In order to qualitatively describe the observed time evolution of the grating reflectivity we propose a three step model for the energy release. In a first step, fast rotational state redistribution following the excitation process leads to the generation of sound waves and a stationary thermal grating. The second step is governed by electronic quenching and results in vibrationally excited O2(IAg) and CO 2 [13] with small energy release. In the last step, the vibrational energy of the products is converted to translational energy.
74
B. Hemmerling et al./ Chemical Physics Letters 256 (1996) 71-76
There are still some further characteristic features of the temporal evolution of the grating reflectivity, depicted in Fig. 2b, which have to be explained. Compared to the pure electrostrictive signal obtained under non.resonant conditions the phase of the oscillating part of the signal is shifted by less than a half period when the excitation laser is tuned to a molecular resonance. This can be caused by a fast negative energy release acounted for by step 1 in our energy release model. Positive energy release would result in a phase shift of more than a half period. The density change due to electrostriction is proportional to sin(2-rr t/Tg), the density variation caused by fast thermalization of absorbed laser energy is proportional to cos(2"rrt/Tg)- l, with negative proportionality constant for negative energy release, see Ref. [11]. Furthermore, the signal starts with a sharp peak coincident in time with the grating excitation pulse. Its leading edge has a duration of about 10 ns and the peak is not in period to the later part of the signal. Electrostriction as well as fast thermalization fail to explain this sharp peak at the begin of the temporal evolution of the grating reflectivity. Therefore, we assume a contribution to the grating reflectivity which results from the induced population change. In combination with the fast energy release of step 1 in our energy release model, this population grating allows a correct description of the leading peak, the phase shift of the oscillating part of the signal at resonance, the intensity ratio of subsequent peaks, and the valley in the signal between the oscillating part and the broad unstructured hump. The slow energy release during the second and the third step in our model accounts for the generation of a stationary thermal grating. It decays by heat conduction and leads to the broad unstructured hump. A more detailed analysis of the temporal evolution of the grating reflectivity is underway and will be the subject of a forthcoming paper. Wavelength scans of the excitation laser have been carried out in pure oxygen at a pressure of 1 bar covering the spectral range of the RR and the RQ branch in the 0 - 0 band of the b l~g *--X 3 ~ ; system. Two distinct time intervals have been used for the signal integration. The first interval starts 20 ns before the excitation pulses and lasts 470 ns, hence covering most of the oscillatory part of the signal. The second interval starts 3.45 Ixs after occur-
1.0.
12121119117 1115 113 111 19 121119117 115 113 Ill ¢l •~.
19
17 17
15 15
]3 13
11 I1 RQ
0.5
u~
0.0 759.5
760
760.5
761
761.5
762
wavelength / nm
1.0 nR
h 32119 l i i 17 ~[15 113 i 11 i 9
==
121119117 115 113 Ill
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19
17 17
15 15
13 13
11 I1 RQ
0.5
m ¢: 131)
759.5
760
760.5
761
761.5
762
wavelength / nrn
Fig. 3. (a) Absorption spectrum (upper trace) and spectrum derived from the oscillatingpart of the LIG signal (lowertrace). (b) Spectrumderivedfrom the unstructuredpart of the LIG signal (in pure oxygen).
rence of the excitation pulses and ends 100 ns later thus capturing the maximum of the unstructured component of the signal. Fig. 3a shows the spectrum derived from the oscillating part of the LIG signal (lower trace) in the wavelength range 759.95 to 761.50 nm together with the simultaneously recorded absorption spectrum (upper trace). Fig. 3b depicts the spectrum derived from the non-oscillating part of the LIG signal. The spectra are normalised to the laser power and a Gaussian with a FWHM of 0.02 cm-~ has been employed as numerical filter of the single-pulse data. All the lines have been identified using the line positions measured by Babcock and Herzberg [14]. The oscillating part of the signal is strongly enhanced in case of molecular resonance. For instance, the signal obtained at the RQ(11) line is
B. Hemmerling et al. / Chemical Physics Letters 256 (1996) 71-76
40 times stronger than the nonresonant electrostricfive signal which appears as constant background in Fig. 3a. The spectrum derived from the non-oscillating part of the laser-induced grating signal is free of background. However, due to the low self-quenching efficiency of oxygen in b l e g the signal is about 30 times smaller than the oscillating part of the signal resulting in a rather poor signal to noise ratio. All the line positions in the absorption spectrum and in the spectra derived form the oscillating and the non oscillating part of the LIG signal agree with each other within the measurement accuracy of __0.01 cm -1. The measured linewidth of 0.1 cm -I (FWHM) of the absorption lines agrees with literature values [15] and is mainly attributed to pressure broadening. The LIG spectra have a linewidth of about 0.07 cm- l (FWHM). The addition of an efficient quencher to the oxygen sample leads to a strong enhancement of the thermal signal. In order to demonstrate the sensitivity of the LIG technique, we looked for lines of the isotopic molecule 160~80 occurring in natural 02 with an abundance of 0.4%. For that purpose, a water partial pressure of 0.018 bar corresponding to the vapour pressure at room temperature was established in a cell containing oxygen at a pressure of 1 bar. In 160180 the transition frequencies are shifted
75
to higher frequencies compared to the frequencies in 1602. In the spectral range 765.6 to 763.8 nm, it was ~ossible to resolve the transitions PP(17), PP(15), P(13), and PP(11) of the 0-0 band in the b l~g X 3~g system of 160180. Most of the other isotopic lines are blended by the wings of the strong transitions belonging to the homonuclear molecule. Fig. 4 shows a part of the spectrum featuring the PP(ll) line which is by a factor of more than 5 × 104 smaller than the neighbouring P P ( 9 ) and PQ(1 l ) line of the homonuclear molecule.
5. Conclusions The temporal development of LIGs, excited via transitions in the red system of molecular oxygen, has been studied. Taking into account contributions to the grating reflectivity resulting from changes in the population distribution, from electrostriction, and from fast and slow release of internal energy the observed signals can be explained. Besides an understanding of the different formation mechanism of LIGs such gratings can provide a valuable tool for the study of molecular energy transfer processes. Furthermore, we showed that molecular transitions which are difficult to detect by laser-induced fluorescence can be investigated using the LIG technique.
1.0
Acknowledgement x 1000
We thank the Swiss Federal Office of Energy (BEW) for financial support.
c
e~ t. tg
0.5
m Ig t-
References
Ot
0.0
J
763.6
763.7
763.8
763.9
764.0
wavelength / nm
Fig. 4. Part of the LIG spectrum obtained in a mixture of 1 bar 02 and 0.018 bar H20. For the middle part of the spectrum, the signal detection sensitivity is enhanced by a factor of 1000. The transition PP(I 1) of the 0 - 0 band in the b IX~- *-- X 3Xg system of 16Ot80 is marked.
[1] K.A. Nelson, D.R. Lutz, M.D. Fayer and L. Madison, Phys. Rev. B 24 (1981) 3261. [2] B. Hemmerling and A. Stampanoni-Panariello, Appl. Phys. B 57 (1993) 281. [3] D.E. Govoni, J.A. Booze, A. Sinha and F.F. Crim. Chem. Phys. Letters 216 (1993) 525. [4] E.B. Cummings, Opt. Letters 19 (1994) 1361. [5] A. Stampanoni-Panariello, B. Hemmerling and W. Hubschmid, Phys. Rev. A 51 (1995) 655. [6] R.L. Abrams, J.F. Lam, R.C. Lind, D.G. Steel and P.F. Liao,
76
[7] [8] [9] [10]
B. Hemmerling et al. / Chemical Physics Letters 256 (1996) 71-76
in: Optical phase conjugation, ed. R.A. Fisher (Academic Press, New York, 1983). A. Dreizler, T. Dreier and J. Wolfrum, Chem. Phys. Letters 233 (1995) 525. P.M. Danehy, P.H. Paul and R.L. Farrow, J. Opt. Soc. Am. B 12 (1995) 1564. S. Williams, L.A. Rahn, P.H. Paul, J.W. Forsman and R.N. Zare, Opt. Letters 19 (1994) 1681. H.J. Eichler, P. Gilnter and D.W. Pohl, Laser induced dynamic gratings (Springer, Berlin, 1986).
[11] W. Hubschmid, B. Hemmerling and A. StampanoniPanariello, J. Opt. Soc. Am. B 12 (1995) 1850. [12] R. Wayne, in: Singlet 02, Vol. 1, Physical-chemical aspects, ed. A.A. Frimer (CRC Press, Boca Raton, FL, 1985) p. 123. [13] R. Wayne, in: Singlet 02, Vol. 1, Physical-chemical aspects, ed. A.A. Frimer (CRC Press, Inc. Boca Raton, FL, 1985) p. 130. [14] H.D. Babcock and L. Herzberg, Astrophys. J. 8 (1948) 167. [15] J.H. Miller, R.W. Boese and L.P. Giver, J. Quant. Spectry. Radiat. Transfer 9 (1969) 1507.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 256 (1996) 71-76
Laser-induced gratings in oxygen excited via the b l g(V' = 0) state B. Hemmerling, R. Bombach, W. Hubschmid Paul Scherrer lnstitut, CH-5232 Villigen PSI, Switzerland Received 28 November 1995; in f'mal form 12 March 1996
Abstract
Laser-induced gratings have been generated by exciting transitions in the red system of molecular oxygen. The temporal evolution of the grating reflectivity has been studied at room temperature and atmospheric pressure. Conlributions to the grating reflectivity arise from a change in population distribution caused by absorption, from electrostriction, and from a release of absorbed laser energy in form of heat. Adding some water to the oxygen sample strongly increases the grating reflectivity rendering the laser-induced grating method sensitive enough to detect the isotopic molecule 160180 in its natural abundance.
I. Introduction
Laser-induced gratings (LIGs) are formed in a medium by various resonant and nonresonant mechanisms as response on the spatially sinusoidal intensity pattern generated by the interference of two intersecting laser beams. Some of the mechanisms have been investigated in detail: electrostrictive gratings are generated at any frequency of the excitation beams and have been observed in time resolved transient grating studies in solids and liquids [1] and in gases [2-5]. In molecules or atoms resonant with the frequency of the incident laser light, a spatially modulated population difference between excited and ground state is generated. Such population gratings are dealt with in degenerate four-wave mixing experiments [6]. Eventually, the excited state energy is released as heat leading to thermal gratings [7,8].
Their potential use for diagnostics in flames has been shown in Ref. [9]. The variety of different contributions to the grating reflectivity may render the interpretation of a measured LIG signal difficult. However, there is the possibility to tailor the experiment to select and enhance one of the contributions to the signal in order to allow for a more sensitive species detection. Furthermore, some of the contributions to the grating reflectivity are based on molecular energy transfer. Analysis of the temporal development of the LIG signal may therefore give some insight into molecular kinetics. In order to analyse the mechanisms contributing to LIGs the gratings are generated by exciting lines in the b 1 ~ - ~ X 3~g system of oxygen. The metastable character of the singlet state allows the investigation of slow nonradiative processes. Fur-
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All fights reserved PH S0009-261 4(96)0041 2-5
72
B. Hemmerling et aL / Chemical Physics Letters 256 (1996) 71-76
thermore, energy transfer processes involving singlet states of 0 2 are of great and continuous interest in atmospheric chemistry and biology. Therefore, on the other hand, LIGs may prove to be a useful method to study molecular energy transfer processes.
trum with a path length of 10 m in a cell containing pure oxygen at 1 bar was taken. In order to allow for time-resolved acquisition of single pulse signals, a digitizer (Tektronix TDS 544A) with a bandwidth of 500 MHz and a sampling rate of 109 s - l was employed.
2. Experimental A scheme of the experimental apparatus is shown in Fig. 1. The two grating excitation beams were provided by a single-longitudinal-mode optical parametric oscillator (Continuum HRL-100Z). The pulse length was about 5 ns and the bandwidth is specified to be smaller than 500 MHz. Focused by a lens ( f = 1000 mm), the two excitation beams overlapped each other at the focus which was located in a sample ceil. At this point the total intensity was approximately 10 G W / c m 2. The angle 0 between the two excitation beams was about 1°. A second lens ( f = 1000 mm) was used to focus the counterpropagating beam of a cw Ar + laser (Innova 70-4) reading out the laser-induced grating in a planar backward phase matching geometry. The power of the Ar ÷ laser was 1.7 W running in single line mode at A = 514.5 nm. Oxygen with a purity of 99.5% was used throughout the measurements and no special care was taken to remove residual water from the gas or the sample cell. Synchronously to the recording of the laser-induced grating spectrum, the laser power was monitored and an absorption spec-
I ~
_
.
BS2
_
BS3 ~
t
~
• :
Ar ÷
I
T I I
Fig. 1. Experimental apparatus. BSI, beamsplitter ( R = 10%); BS2 and BS3, beamsplitter (R = 50%); FP, Fabry-Perot interferometer; WM, wave meter; PM, photomultiplier tube; D, digitizer.
3. Results and discussion The diffraction efficiency of a laser-induced grating is, see e.g. Ref. [10], =
+
(1)
where d is the thickness of the grating, A is the wavelength of the excitation laser, and A n and A K are the change of the refractive index and of the absorption coefficient across a fringe of the grating, respectively. If there is no molecular/atomar resonance at the wavelength of the laser reading out the grating, A K is equal to zero and a pure phase grating is obtained. The refractive index modulation can phenomenologically be described by
An= ~ij ANij+ -~ oAT+t-~-fi]r p
(2)
where N/; is the deviation from the thermal population difference between the two molecular levels i and j connected by the frequency of the excitation laser. The first term in Eq. (2) accounts for the influence of the population transfer caused by the excitation laser on the refractive index at the wavelength of the read-out laser. Once the absorbed energy is thermalized the refractive index is a function of (local) thermodynamic quantities. In a gas the thermo-optic coefficient (3n/ST) is usually small and can be neglected in comparison to the change of the refractive index caused by the variation in density. The density change effected by electrostriction and by thermalization of absorbed laser energy can be calculated by solving the linearized fluid dynamic equations, see e.g. Ref. [11]. The solution of these equations is a stationary density wave at constant pressure superimposed on a standing acoustic wave
B. Hemmerling et al./ Chemical Physics Letters 256 (1996) 71-76
with contributions arising from electrostriction and heat release. Viscosity and heat conduction damp the acoustic wave. The stationary wave is damped by heat conduction alone. The oscillation period of the grating reflectivity is Tg = h / [ 2 v sin(0/2)] where v is the adiabatic sound velocity. If no internal energy is released in form of heat, the amplitude of the stationary density wave is much smaller than the one of the density change of the acoustic wave. In this case of pure electrostriction, the density oscillates around its undisturbed value leading to an oscillation period of the grating reflectivity of Tg/2. The properties of LIGs formed by thermalization of absorbed laser energy depend on the time constant T~ of the energy release. Fast energy release (Tg >> 2"rrTr) generates a sound wave and a stationary density wave with equal amplitude. Slow energy release (Tg << 2~rT~) favours the generation of the stationary density wave whereas the development of sound waves is hindered due to the destructive superposition of sound waves generated at different times. Fig. 2 shows the temporal behaviour of the reflectivity for a grating generated by a single pulse of the excitation laser in a mixture of 1 bar oxygen and 0.05 bar carbon dioxide. The wavelength of the laser was tuned to the PP(9) line in the 0 - 0 band of the b lX~- ,_ X 3Xg system of oxygen. The appearance of the oscillating part of the signal resembles the one of a signal resulting from a superposition of a thermal and an electrostrictive grating. After the oscillating part has vanished, due to damping and because the sound waves have left the interaction volume, a broad unstructured hump appears. In a mixture of 1 bar oxygen and 0.05 bar carbon dioxide the time constant for quenching of O 2 in the b 1Eg+ ( v ' = 0) state is 3.5 I~s [12]. This is too slow to excite an acoustic amplitude from the thermalization of absorbed laser energy which is comparable to the one excited by electrostriction. A calculation shows that for slow thermalization the reflectivity of the oscillating part of the grating is reduced by a factor of the o r d e r TgTd/4"rrTr 2 ( T d is the dissipation time of the grating) compared to fast thermalization with the same energy release. Therefore, the small amount of internal energy converted t o / f r o m translation energy which accompanies rotational state redistribution within the ensemble of molecules involved in the excitation process can generate a stronger
73
1.0
c
z.
m
0.5
c
0.0 i
t
i
I
0.0
5.0
10.0
15.C
time / tJs 1.0
b e-
o.s
g 0.0 0.o
o.s
1.0
1.s
time / /JS
Fig. 2. (a) Single-pulse LIG signal obtained in a mixture of 1 bar 02 and 0.05 bar CO 2. (b) The first part of the signal with higher temporal resolution.
oscillating grating than the energy release which results from subsequently following slow energy transfer processes. On the other hand, the energy release by electronic quenching is too fast to be responsible for the broad unstructured hump visible in Fig. 2b at about 5 ixs. In order to qualitatively describe the observed time evolution of the grating reflectivity we propose a three step model for the energy release. In a first step, fast rotational state redistribution following the excitation process leads to the generation of sound waves and a stationary thermal grating. The second step is governed by electronic quenching and results in vibrationally excited O2(IAg) and CO 2 [13] with small energy release. In the last step, the vibrational energy of the products is converted to translational energy.
74
B. Hemmerling et al./ Chemical Physics Letters 256 (1996) 71-76
There are still some further characteristic features of the temporal evolution of the grating reflectivity, depicted in Fig. 2b, which have to be explained. Compared to the pure electrostrictive signal obtained under non.resonant conditions the phase of the oscillating part of the signal is shifted by less than a half period when the excitation laser is tuned to a molecular resonance. This can be caused by a fast negative energy release acounted for by step 1 in our energy release model. Positive energy release would result in a phase shift of more than a half period. The density change due to electrostriction is proportional to sin(2-rr t/Tg), the density variation caused by fast thermalization of absorbed laser energy is proportional to cos(2"rrt/Tg)- l, with negative proportionality constant for negative energy release, see Ref. [11]. Furthermore, the signal starts with a sharp peak coincident in time with the grating excitation pulse. Its leading edge has a duration of about 10 ns and the peak is not in period to the later part of the signal. Electrostriction as well as fast thermalization fail to explain this sharp peak at the begin of the temporal evolution of the grating reflectivity. Therefore, we assume a contribution to the grating reflectivity which results from the induced population change. In combination with the fast energy release of step 1 in our energy release model, this population grating allows a correct description of the leading peak, the phase shift of the oscillating part of the signal at resonance, the intensity ratio of subsequent peaks, and the valley in the signal between the oscillating part and the broad unstructured hump. The slow energy release during the second and the third step in our model accounts for the generation of a stationary thermal grating. It decays by heat conduction and leads to the broad unstructured hump. A more detailed analysis of the temporal evolution of the grating reflectivity is underway and will be the subject of a forthcoming paper. Wavelength scans of the excitation laser have been carried out in pure oxygen at a pressure of 1 bar covering the spectral range of the RR and the RQ branch in the 0 - 0 band of the b l~g *--X 3 ~ ; system. Two distinct time intervals have been used for the signal integration. The first interval starts 20 ns before the excitation pulses and lasts 470 ns, hence covering most of the oscillatory part of the signal. The second interval starts 3.45 Ixs after occur-
1.0.
12121119117 1115 113 111 19 121119117 115 113 Ill ¢l •~.
19
17 17
15 15
]3 13
11 I1 RQ
0.5
u~
0.0 759.5
760
760.5
761
761.5
762
wavelength / nm
1.0 nR
h 32119 l i i 17 ~[15 113 i 11 i 9
==
121119117 115 113 Ill
..o ,~
19
17 17
15 15
13 13
11 I1 RQ
0.5
m ¢: 131)
759.5
760
760.5
761
761.5
762
wavelength / nrn
Fig. 3. (a) Absorption spectrum (upper trace) and spectrum derived from the oscillatingpart of the LIG signal (lowertrace). (b) Spectrumderivedfrom the unstructuredpart of the LIG signal (in pure oxygen).
rence of the excitation pulses and ends 100 ns later thus capturing the maximum of the unstructured component of the signal. Fig. 3a shows the spectrum derived from the oscillating part of the LIG signal (lower trace) in the wavelength range 759.95 to 761.50 nm together with the simultaneously recorded absorption spectrum (upper trace). Fig. 3b depicts the spectrum derived from the non-oscillating part of the LIG signal. The spectra are normalised to the laser power and a Gaussian with a FWHM of 0.02 cm-~ has been employed as numerical filter of the single-pulse data. All the lines have been identified using the line positions measured by Babcock and Herzberg [14]. The oscillating part of the signal is strongly enhanced in case of molecular resonance. For instance, the signal obtained at the RQ(11) line is
B. Hemmerling et al. / Chemical Physics Letters 256 (1996) 71-76
40 times stronger than the nonresonant electrostricfive signal which appears as constant background in Fig. 3a. The spectrum derived from the non-oscillating part of the laser-induced grating signal is free of background. However, due to the low self-quenching efficiency of oxygen in b l e g the signal is about 30 times smaller than the oscillating part of the signal resulting in a rather poor signal to noise ratio. All the line positions in the absorption spectrum and in the spectra derived form the oscillating and the non oscillating part of the LIG signal agree with each other within the measurement accuracy of __0.01 cm -1. The measured linewidth of 0.1 cm -I (FWHM) of the absorption lines agrees with literature values [15] and is mainly attributed to pressure broadening. The LIG spectra have a linewidth of about 0.07 cm- l (FWHM). The addition of an efficient quencher to the oxygen sample leads to a strong enhancement of the thermal signal. In order to demonstrate the sensitivity of the LIG technique, we looked for lines of the isotopic molecule 160~80 occurring in natural 02 with an abundance of 0.4%. For that purpose, a water partial pressure of 0.018 bar corresponding to the vapour pressure at room temperature was established in a cell containing oxygen at a pressure of 1 bar. In 160180 the transition frequencies are shifted
75
to higher frequencies compared to the frequencies in 1602. In the spectral range 765.6 to 763.8 nm, it was ~ossible to resolve the transitions PP(17), PP(15), P(13), and PP(11) of the 0-0 band in the b l~g X 3~g system of 160180. Most of the other isotopic lines are blended by the wings of the strong transitions belonging to the homonuclear molecule. Fig. 4 shows a part of the spectrum featuring the PP(ll) line which is by a factor of more than 5 × 104 smaller than the neighbouring P P ( 9 ) and PQ(1 l ) line of the homonuclear molecule.
5. Conclusions The temporal development of LIGs, excited via transitions in the red system of molecular oxygen, has been studied. Taking into account contributions to the grating reflectivity resulting from changes in the population distribution, from electrostriction, and from fast and slow release of internal energy the observed signals can be explained. Besides an understanding of the different formation mechanism of LIGs such gratings can provide a valuable tool for the study of molecular energy transfer processes. Furthermore, we showed that molecular transitions which are difficult to detect by laser-induced fluorescence can be investigated using the LIG technique.
1.0
Acknowledgement x 1000
We thank the Swiss Federal Office of Energy (BEW) for financial support.
c
e~ t. tg
0.5
m Ig t-
References
Ot
0.0
J
763.6
763.7
763.8
763.9
764.0
wavelength / nm
Fig. 4. Part of the LIG spectrum obtained in a mixture of 1 bar 02 and 0.018 bar H20. For the middle part of the spectrum, the signal detection sensitivity is enhanced by a factor of 1000. The transition PP(I 1) of the 0 - 0 band in the b IX~- *-- X 3Xg system of 16Ot80 is marked.
[1] K.A. Nelson, D.R. Lutz, M.D. Fayer and L. Madison, Phys. Rev. B 24 (1981) 3261. [2] B. Hemmerling and A. Stampanoni-Panariello, Appl. Phys. B 57 (1993) 281. [3] D.E. Govoni, J.A. Booze, A. Sinha and F.F. Crim. Chem. Phys. Letters 216 (1993) 525. [4] E.B. Cummings, Opt. Letters 19 (1994) 1361. [5] A. Stampanoni-Panariello, B. Hemmerling and W. Hubschmid, Phys. Rev. A 51 (1995) 655. [6] R.L. Abrams, J.F. Lam, R.C. Lind, D.G. Steel and P.F. Liao,
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[7] [8] [9] [10]
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in: Optical phase conjugation, ed. R.A. Fisher (Academic Press, New York, 1983). A. Dreizler, T. Dreier and J. Wolfrum, Chem. Phys. Letters 233 (1995) 525. P.M. Danehy, P.H. Paul and R.L. Farrow, J. Opt. Soc. Am. B 12 (1995) 1564. S. Williams, L.A. Rahn, P.H. Paul, J.W. Forsman and R.N. Zare, Opt. Letters 19 (1994) 1681. H.J. Eichler, P. Gilnter and D.W. Pohl, Laser induced dynamic gratings (Springer, Berlin, 1986).
[11] W. Hubschmid, B. Hemmerling and A. StampanoniPanariello, J. Opt. Soc. Am. B 12 (1995) 1850. [12] R. Wayne, in: Singlet 02, Vol. 1, Physical-chemical aspects, ed. A.A. Frimer (CRC Press, Boca Raton, FL, 1985) p. 123. [13] R. Wayne, in: Singlet 02, Vol. 1, Physical-chemical aspects, ed. A.A. Frimer (CRC Press, Inc. Boca Raton, FL, 1985) p. 130. [14] H.D. Babcock and L. Herzberg, Astrophys. J. 8 (1948) 167. [15] J.H. Miller, R.W. Boese and L.P. Giver, J. Quant. Spectry. Radiat. Transfer 9 (1969) 1507.