- Email: [email protected]
Thermovision monitoring of gas-phase fluorescence induced by cw CO2 laser radiation
Sprcmchimico
Acta, Vol. 46A, No. 4, pp. 559 - 562.1990.
0584 - 8539/90 $3.00 + 0.00
Printed in Great Britain.
8 1990 Fmgamon Press
THERMOVISION MONITORING OF GAS-PHASE FLUORESCENCE INDUCED BY CW CO2 RADIATION
plc
LASER
J. Krasa, P. EngstX and M. Hor4kX Institute of Physics, ESAV, 180 40 Prague, Czechoslovakia XThe J. HeyrovskjrInstitute of Physical Chemistry and Electrochemistry, ESAV, 182 23 Prague, Czechoslovakia
ABSTRBCT - Applying thermovision monitoring, spatial distribution of gas-phase fluorescence in the wavelength region 2-5.4 um induced by CO laser radiation is displayed. Experimental results demonstrdte that the spat?_ al distribution of the fluorescence is effected not only by the convection in a reaction cell but it also depends on V-T energy transfer relaxation time. Relatively slow relaxation compared to fast convection can cause that dominant portion of excited molecules is quenched on the cell walls. The used thermovision technique allows to determine both the threshold and the steady-state of convection generated by cw CO2 laser beam absorption.
INTRODUCTION
Relaxation orocesses in gaseous polyatomic molecules vibrationally excited by laser radiation result in an increase of the gas temperature. The absorption of a sufficiently powerful laser beam causes not only some nonlinear relaxation urocesses but also a mas convection /l/. As it is well-known, the latter phenO&&n is important foi cw CO laser applications, too. The convection infiuences the distribution of exci.?ed particles inside a reaction cell as well as the temperature distribution and it can cause that the walls are able to effect chemical reactions induced by the cw laser radiation. In principle, it is erroneous to assume that the thermal convection is neglible compared to thermal conduction as it is sometimes presented /2/. It is evident that it should be experimentally verified whether the theoretical assumptions suit experimental conditions which is fundamental for an evaluation of the experiment. As a suitable contactless experimental technique that can solve the above mentioned oroblems can be used a thermovision monitoring of gas-phase fluorescence. Such a monitoring gives information about spatial distribution of the gas fluorescence by means of which it is possible to determine the spatial distribution of excited molecules as well a% the threshould and the steady-state of the convection and eventually the influence of walls on the chemical kinetics. This technique generates more data than other methods using e.g. He-Ne laser beam passing through the reaction cell. Our contribution demonstrates that a standart thermovision technique originally designed for thermal imaging of solids can be applied to monitoring the fluorescence of molecular gases excited by cw CO laser beam. The displayed fluorescence of different gases shows that a max zmum of excited molecules is not commonlv located in a soace of laser beam absorotion (i.e. in the hot zone) but it may be shifted below the cell top. This shifi originates in both the gas convection and the relatively slow V-T relaxation of strongly -_ excited molecules. An effect of the cell upper wall on quenching of the excited molecules is demonstrable for many different gases under similar conditions. APPARATUS For the excitation of vibrational states of molecules a laboratory cw CO2 laser with grating for selection of the rotational-vibrational lines was used. About 20 W of the laser output on one line was obtained. The fluorescence of gases was followed in a prism cell (90x40x50 mm) fitted with two NaCl windows for the 559
J. KRkA et cd.
560
laser beam and NaCl walls permitting measurements along a path perpendicular to the IR laser beam. The cell was filled with SF6, NH3, CH31, CD31 and C2H4 respectively at a pressure 0.3-10 kPa. A steady state distribution of the vibrationally excited molecules was monitored by using thermovision AGA model 750 equipped with an InSb detector scanning the fluorescence in the region of 2-5.4 um. A horizontal-vertical scanning system directs the IR signal onto the deteL tar, The IR signal was imaged on the monitor screan. The picture size on the monitor was 50 x 50 mm. There are 280 lines per frame with the resolving power of 100 elements per line and scanning speed of 25 Hz. The thermovision AGA model 750 gives the picture of individual isotherms, more precisely isoluxes, i.e. curves of equal light intensity. The thermovision signal S(x.,x.) relates to an integral number of molecules emitting in the range of 2-5.4/d along the cell depht
S(Xi,Xj) =
na+An D / / na
0
5.4pm / nx(Xi~Xjpxk~A) f(x) dh dxkdnX 2pm
where nx is the density of the excited molecules emitting photons at the wavelength h, f(h) describes detector spectral sensitivity, D the thickness of the cell and the range (n ,n +An) is the chosen level of detector signal. This simple result could be egtefidedby using an interference filter as a disperse element before the detector. It enabled us to represftntthe ener level space distribution. In addition, the exact picture of n (x.,x.,x A? would be obtained by a thomographic method of reconstruction from ?la$ plfhturestaken from several directions. EXPERIMENTAL A period necessary for establishing a steady-state flow pattern during the absorption of the cw CO laser beam under our experimental conditions was for all gases comparable, app$oaching 1 s. It agrees with experimental results presented in /3/. A very rapid formation of a detectable fluorescence zone at the entrance of the laser beam into the cell was observed in SF Fig.1 recorded by the theron of gravitational field) movision camera from above the cell (in the direct4' shows the axial symmetry of all three isoluxes around the axis of the absorbed beam (pressure of SF 1.3 kPa; unfoclfsedQser beam, its diameter 10 mm output power 10 W, the PC269 line of the 00 1 +lO 0 transition of CO moleculeI. The record obtained from the cell side under the same conditions !?. s shown in Fig.2; it represents breakdown of the axial symmetry of the isoluxes due to a convection of SF6 is appearent. The record of the isolux lines in C2H (8 kPa, the P(14) line, 10 W unfocused beam) taken against the passage of tde laser beam is shown in Fig.3. The total IR radiation (wavelength region 2-5.4 urn)of a horizontal layer of an infinitesimal thickness has not the maximum 4n the hot zone center but in a region under the cell ceiling contrary to SF Similar spatial distributions of the fluorescence were observed in CD I CH ?" C F NH also, A set of isoluxes recorded in CD I (4 kPa the P(14) l&e ?0'W)6ig'sho&n in Fig.4. The maximum of the side f&orescenAe lies over the'hot zone. The isoluxes of a lower level crossing the absorbed laser beam are not illustrated, To obtain more informations about a possible mechanism shifting the maximum fluorescence outside the hot zone the laser beam power density was increased inserting a focusing lens, A corresponding record of the isoluxes is given in Fig.5. The focusing lens of f=0.25 m was located in front of the cell window at the distance 0.2 m. The increase of the power density leads to a new spatial distribution of the fluorescence intensity. The unexpected distribution of the fluorescence of CD I makes clear that the multitude of the detectable excited molecules decrease2 from the centre of the laser beam upward to the cell upper wall. After reaching a minimum the fluorescence increases to the absolute maximum located approximately 8 mm under the cell upper wall. This distance is nearly the same as in the case of the unfocused laser beam. DISCUSSION The processes of excitation and relaxation of molecules during the absorption of a cw CO2 laser beam can generate a convection of illuminated gases as schematically illustrated in Fig.6. A process of thermalization cannot be commonly localized into the hot zone. It depends on properties of an used gas, parameters of a laser beam, dimensions and quality of cell walls, respectively. The thermovision technique can record the radiation at the wavelength which would correspond to a minimum two-photon absorption. Such an excited state can
561
Thermovisionmonitoring ofgas-phase fluorescence
Fig.1. ~-Set of visualized IR isoluxks (2-5.4 urn)of the hot zone in SF (pf;esslf;e 1.3 kPa, PC261 line of6the 00 l+lO 0 transition, 10 W unfocused CO, laser beam entering into the cell frbm the right side, view from above).
Fig.2. Hot zone in SF (1.3 kPa, P(26) line, 10 W unfoc8 sed laser beam, lateral view).
___________~~~-~_~-~--~~~\
r-----y
.___._
#’
,\
‘.
._.-.-.---._.
_..
\\
c ----- -._._.__ ----*--“-~- ._.- i
:
5*8
-a__
________e-e-e
-___
,;
c-0
5
Fig.4. Hot zone in CD I (4 kPa, P(14) line, 10 W unfocused laser b 2 am, lateral view, isoluxes traced from photograph).
Fig.3. Hot zone m ethylene (8 kPa, P(14) line, 10 W unfocused laser beam, frontal view).
I Fig.5. Hot zone in CD I (4 kPa, P(14) line, 10 W laser beam focuse2 by 250 mm lens; the focus is located about 1 cm behind the input NaCl window).
6
Y
Laser beam
Y
Fig.6. Schematic view of the experiment and of the convection induced by cw CO2 laser beam.
562
J.
%&A
et al.
be produced either by vibrational energy transfer or by the process of intramolecular relaxation with a subsequent photon absorption. Another possibility is represented by the processes of intense V-T energy transfer and energy level thermalization. The different behaviour of the gases relates to their absorption and relaxation properties. Under the given conditions the laser beam was totally absorbed only by SF6 end in the case of the other gases the absorption was lo-40% /4/. The spatial distribution of the fluorescence clearly depend on the used gases as demonstrated in Figs.2 and 5. In the Fig.5 we can see, that the CD I fluorescent space is divided in two parts. In the first one where the laser a eam is absol?bed.both the direct excitation of molecules end the fast V-V energy transfer are dominant and generate higher vibrational levels lying in the spectral range of the detector. The most intensive fluorescence is located in the upper part of the cell above the hot zone. This effect is probably caused by a cumulation of excited molecules in the upper part of the cell. Both the shape end the localization of the 4.5 level isolux in Fig.3 clearly manifest this cumulation and also a deexcitation of molecules on the walls of the cell. The minimum of fluorescence lying above the hot zone can result as a consequence of both the V-T relaxation end the fast convection. This phenomenon was absent in SF6 because of a relatively fast V-T relaxation. In this case the convection only disturbes a symmetry of the hot zone. REFERENCES Bailey R.T. et al.: J. Chem. Phys. 77 (1982) 3453. Sitter R.N.: Spectrochim. Acta 43A (1987) 245. % /3/ Kub4t P., Pola J.: Coil. Czech. Chem. Commun. 50 (1985) 1573. /4/ Rejnek J., Engst P., Jakoubkovd M., Hordk M.: Coil, Czech. Chem. Commun. 50 (1983) 215.
Acta, Vol. 46A, No. 4, pp. 559 - 562.1990.
0584 - 8539/90 $3.00 + 0.00
Printed in Great Britain.
8 1990 Fmgamon Press
THERMOVISION MONITORING OF GAS-PHASE FLUORESCENCE INDUCED BY CW CO2 RADIATION
plc
LASER
J. Krasa, P. EngstX and M. Hor4kX Institute of Physics, ESAV, 180 40 Prague, Czechoslovakia XThe J. HeyrovskjrInstitute of Physical Chemistry and Electrochemistry, ESAV, 182 23 Prague, Czechoslovakia
ABSTRBCT - Applying thermovision monitoring, spatial distribution of gas-phase fluorescence in the wavelength region 2-5.4 um induced by CO laser radiation is displayed. Experimental results demonstrdte that the spat?_ al distribution of the fluorescence is effected not only by the convection in a reaction cell but it also depends on V-T energy transfer relaxation time. Relatively slow relaxation compared to fast convection can cause that dominant portion of excited molecules is quenched on the cell walls. The used thermovision technique allows to determine both the threshold and the steady-state of convection generated by cw CO2 laser beam absorption.
INTRODUCTION
Relaxation orocesses in gaseous polyatomic molecules vibrationally excited by laser radiation result in an increase of the gas temperature. The absorption of a sufficiently powerful laser beam causes not only some nonlinear relaxation urocesses but also a mas convection /l/. As it is well-known, the latter phenO&&n is important foi cw CO laser applications, too. The convection infiuences the distribution of exci.?ed particles inside a reaction cell as well as the temperature distribution and it can cause that the walls are able to effect chemical reactions induced by the cw laser radiation. In principle, it is erroneous to assume that the thermal convection is neglible compared to thermal conduction as it is sometimes presented /2/. It is evident that it should be experimentally verified whether the theoretical assumptions suit experimental conditions which is fundamental for an evaluation of the experiment. As a suitable contactless experimental technique that can solve the above mentioned oroblems can be used a thermovision monitoring of gas-phase fluorescence. Such a monitoring gives information about spatial distribution of the gas fluorescence by means of which it is possible to determine the spatial distribution of excited molecules as well a% the threshould and the steady-state of the convection and eventually the influence of walls on the chemical kinetics. This technique generates more data than other methods using e.g. He-Ne laser beam passing through the reaction cell. Our contribution demonstrates that a standart thermovision technique originally designed for thermal imaging of solids can be applied to monitoring the fluorescence of molecular gases excited by cw CO laser beam. The displayed fluorescence of different gases shows that a max zmum of excited molecules is not commonlv located in a soace of laser beam absorotion (i.e. in the hot zone) but it may be shifted below the cell top. This shifi originates in both the gas convection and the relatively slow V-T relaxation of strongly -_ excited molecules. An effect of the cell upper wall on quenching of the excited molecules is demonstrable for many different gases under similar conditions. APPARATUS For the excitation of vibrational states of molecules a laboratory cw CO2 laser with grating for selection of the rotational-vibrational lines was used. About 20 W of the laser output on one line was obtained. The fluorescence of gases was followed in a prism cell (90x40x50 mm) fitted with two NaCl windows for the 559
J. KRkA et cd.
560
laser beam and NaCl walls permitting measurements along a path perpendicular to the IR laser beam. The cell was filled with SF6, NH3, CH31, CD31 and C2H4 respectively at a pressure 0.3-10 kPa. A steady state distribution of the vibrationally excited molecules was monitored by using thermovision AGA model 750 equipped with an InSb detector scanning the fluorescence in the region of 2-5.4 um. A horizontal-vertical scanning system directs the IR signal onto the deteL tar, The IR signal was imaged on the monitor screan. The picture size on the monitor was 50 x 50 mm. There are 280 lines per frame with the resolving power of 100 elements per line and scanning speed of 25 Hz. The thermovision AGA model 750 gives the picture of individual isotherms, more precisely isoluxes, i.e. curves of equal light intensity. The thermovision signal S(x.,x.) relates to an integral number of molecules emitting in the range of 2-5.4/d along the cell depht
S(Xi,Xj) =
na+An D / / na
0
5.4pm / nx(Xi~Xjpxk~A) f(x) dh dxkdnX 2pm
where nx is the density of the excited molecules emitting photons at the wavelength h, f(h) describes detector spectral sensitivity, D the thickness of the cell and the range (n ,n +An) is the chosen level of detector signal. This simple result could be egtefidedby using an interference filter as a disperse element before the detector. It enabled us to represftntthe ener level space distribution. In addition, the exact picture of n (x.,x.,x A? would be obtained by a thomographic method of reconstruction from ?la$ plfhturestaken from several directions. EXPERIMENTAL A period necessary for establishing a steady-state flow pattern during the absorption of the cw CO laser beam under our experimental conditions was for all gases comparable, app$oaching 1 s. It agrees with experimental results presented in /3/. A very rapid formation of a detectable fluorescence zone at the entrance of the laser beam into the cell was observed in SF Fig.1 recorded by the theron of gravitational field) movision camera from above the cell (in the direct4' shows the axial symmetry of all three isoluxes around the axis of the absorbed beam (pressure of SF 1.3 kPa; unfoclfsedQser beam, its diameter 10 mm output power 10 W, the PC269 line of the 00 1 +lO 0 transition of CO moleculeI. The record obtained from the cell side under the same conditions !?. s shown in Fig.2; it represents breakdown of the axial symmetry of the isoluxes due to a convection of SF6 is appearent. The record of the isolux lines in C2H (8 kPa, the P(14) line, 10 W unfocused beam) taken against the passage of tde laser beam is shown in Fig.3. The total IR radiation (wavelength region 2-5.4 urn)of a horizontal layer of an infinitesimal thickness has not the maximum 4n the hot zone center but in a region under the cell ceiling contrary to SF Similar spatial distributions of the fluorescence were observed in CD I CH ?" C F NH also, A set of isoluxes recorded in CD I (4 kPa the P(14) l&e ?0'W)6ig'sho&n in Fig.4. The maximum of the side f&orescenAe lies over the'hot zone. The isoluxes of a lower level crossing the absorbed laser beam are not illustrated, To obtain more informations about a possible mechanism shifting the maximum fluorescence outside the hot zone the laser beam power density was increased inserting a focusing lens, A corresponding record of the isoluxes is given in Fig.5. The focusing lens of f=0.25 m was located in front of the cell window at the distance 0.2 m. The increase of the power density leads to a new spatial distribution of the fluorescence intensity. The unexpected distribution of the fluorescence of CD I makes clear that the multitude of the detectable excited molecules decrease2 from the centre of the laser beam upward to the cell upper wall. After reaching a minimum the fluorescence increases to the absolute maximum located approximately 8 mm under the cell upper wall. This distance is nearly the same as in the case of the unfocused laser beam. DISCUSSION The processes of excitation and relaxation of molecules during the absorption of a cw CO2 laser beam can generate a convection of illuminated gases as schematically illustrated in Fig.6. A process of thermalization cannot be commonly localized into the hot zone. It depends on properties of an used gas, parameters of a laser beam, dimensions and quality of cell walls, respectively. The thermovision technique can record the radiation at the wavelength which would correspond to a minimum two-photon absorption. Such an excited state can
561
Thermovisionmonitoring ofgas-phase fluorescence
Fig.1. ~-Set of visualized IR isoluxks (2-5.4 urn)of the hot zone in SF (pf;esslf;e 1.3 kPa, PC261 line of6the 00 l+lO 0 transition, 10 W unfocused CO, laser beam entering into the cell frbm the right side, view from above).
Fig.2. Hot zone in SF (1.3 kPa, P(26) line, 10 W unfoc8 sed laser beam, lateral view).
___________~~~-~_~-~--~~~\
r-----y
.___._
#’
,\
‘.
._.-.-.---._.
_..
\\
c ----- -._._.__ ----*--“-~- ._.- i
:
5*8
-a__
________e-e-e
-___
,;
c-0
5
Fig.4. Hot zone in CD I (4 kPa, P(14) line, 10 W unfocused laser b 2 am, lateral view, isoluxes traced from photograph).
Fig.3. Hot zone m ethylene (8 kPa, P(14) line, 10 W unfocused laser beam, frontal view).
I Fig.5. Hot zone in CD I (4 kPa, P(14) line, 10 W laser beam focuse2 by 250 mm lens; the focus is located about 1 cm behind the input NaCl window).
6
Y
Laser beam
Y
Fig.6. Schematic view of the experiment and of the convection induced by cw CO2 laser beam.
562
J.
%&A
et al.
be produced either by vibrational energy transfer or by the process of intramolecular relaxation with a subsequent photon absorption. Another possibility is represented by the processes of intense V-T energy transfer and energy level thermalization. The different behaviour of the gases relates to their absorption and relaxation properties. Under the given conditions the laser beam was totally absorbed only by SF6 end in the case of the other gases the absorption was lo-40% /4/. The spatial distribution of the fluorescence clearly depend on the used gases as demonstrated in Figs.2 and 5. In the Fig.5 we can see, that the CD I fluorescent space is divided in two parts. In the first one where the laser a eam is absol?bed.both the direct excitation of molecules end the fast V-V energy transfer are dominant and generate higher vibrational levels lying in the spectral range of the detector. The most intensive fluorescence is located in the upper part of the cell above the hot zone. This effect is probably caused by a cumulation of excited molecules in the upper part of the cell. Both the shape end the localization of the 4.5 level isolux in Fig.3 clearly manifest this cumulation and also a deexcitation of molecules on the walls of the cell. The minimum of fluorescence lying above the hot zone can result as a consequence of both the V-T relaxation end the fast convection. This phenomenon was absent in SF6 because of a relatively fast V-T relaxation. In this case the convection only disturbes a symmetry of the hot zone. REFERENCES Bailey R.T. et al.: J. Chem. Phys. 77 (1982) 3453. Sitter R.N.: Spectrochim. Acta 43A (1987) 245. % /3/ Kub4t P., Pola J.: Coil. Czech. Chem. Commun. 50 (1985) 1573. /4/ Rejnek J., Engst P., Jakoubkovd M., Hordk M.: Coil, Czech. Chem. Commun. 50 (1983) 215.