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Photonucleation in a supersaturated vapor
Volume 59, number
1
CHEMICAL
PHYSICS
LE’ITERS
1 November 1978
PHOTONUCLEATION IN A SUPERSATURATED VAPOR B. CORDIER and P_ PAPON Luboratoire de Rkonance Magnefique. Equipe de Recherche associ&eau C.N.R.S.. Ecole Suphieure de PI2ysique et de CWmie IndustrieIIes. 75005 PZU& France Received 10 July 1978 We report experimental results of the investigation of the photonuckation process in a diffusion chamber. We have shovm that nucleation can be induced in a supersaturated vapor of Ca in the presence of a small concentration of C!* at A = 4880 A, 5145 A using laser light. The process is strong& dependent upon the intensity and wavelength of light and upon the supersturation of the vapor.
1. Iutroduction Recent experiients performed in various conditions have revealed the possibility of inducing the gas-livid transition by light. Eventual photonucleation initiated in a vapor by a resonant radiation has been widely discussed by Gudzenko [I] and by Mlikhviladze and Sarychev [2] _ They showed that radiation which is absorbed by a vapor can change the pair interaction potentials between the moiecules and thus the parameters of the equation of state for the gas-liquid system. The coexistence curve and the spinodal might be shifted under illumination and a supersaturated vapor might thus nucleate_ Their calculations seem to account for a photonucleation phenomenon observed in iodine [3]_ Katz axedCo1 [4] have observed that on irradiation with light of wavelength shorter than 420 nm certain organic compounds, even at very low concentration, are able to cause the nucleation of supersaturated vapors. They interpreted their results with a mechanism in which nuclei with only a few photoexcited molecules were able to initiate the nucleation process within the vapor. The same authors have recently reported similar experiments on the photoinduced nucleation of water vapor [S] and they have shown that it is an intrinsic property of water, the phenomenon occurring even in the absence of an impurity. Many other experiments have been performed in the field of photochemically generated aerosols, they
have particularly shown that ultraviolet light can produce aerosols in vapor systems, the chemical reaction being induced by moiecules or radicals in excited states [6,7]. Chemical reactions in the gas phase can also be stimulated by laser light following the same mechanisms as reported for example by I-Ierman and Co1 IS] _ Finally, isotope-selective dissociation of molecules by laser radiation in the infrared spectrum have tended to be extensively studied in various systems as, for example, CC14 and CF,Cl? 19-l 1] . Isotopic enrichment in some atomic species is one of the motivations behind such studies. We wish to report preliminary results of an investigation of nucleation of supersaturated CC4 induced by light in the visible part of the spectrum.
2. Experiment21 results We studied the nucleation phenomenon in a supersaturated vapor with a thermal diffusion cloud chamber. This chamber 1121 can maintain a vapor in a state of continuous supersaturation and consists of two horizontal and circular copper plates of 520 mm diameter separated by a stainless steel ring 50 mm high and 450 mm of inner diameter with four glass windows- The lower plate is maintained at a high temperature and the upper plate at a low temperature; one-dimensional heat and mass flows settle between the two plates. A computer solution of the 113
vobmle
SF_ number I
CHEitfKXL
PHYSICS
heat conduction and mass diffusion equations gives the temperature and partial pressure profles within the vapor_ CC4 is introduced in the chamber with a hght carrier gas as helium. With the experimental device we studied first the vapor system by Raylcigh light scattering using an argon ion laser as a light source. Preliminary measurements performed by Barbut [13] showed an anornalousIy Iarge intensity of light scattered by the vapor at iow supersaturation ratio- Carefui investigation revealed that the faser be&n was inducing nucleation. A thorough examination of the phenomenon has since revealed that the photonucieation in CC4 was due to the presence of impurities wi?ich were proved to be Ciz. We have shown that pure CC14 in presence of light does not nucleate until the supersaturation reaches its critical value; conversely one can induce nucleation by light in CC14 in the presence of Ci2 at low concentration even at iow supersaturation. We have thoroughly investigated this photonucleation phenomenon in the Ci&-cg2-He vapor system. The heat and mass transfer equations have been modified to take into account the presence of Ciz fI4] and the pressure and temperature profdes recalculated_ The copper plates have been protected against corrosion. Experiments have been performed at various frequencies @ = 4880 & 5 14.5A _ . _) and at tiifferent light intensities_ An experiment performed with an He-Ne Iaser (X = 6328 L%,35 mW) showed LO effect of ii& on nucleation at various supersaturation ratios. We chose to measure a parameter which we clrll the critical intensity_ It represents the light intensity which is needed to produce a nuckation rate of one t0 tW0 drO2.S CM-’ S- ’ in two minutes (the time interval is chosen constant for all experiments) following iJiurnination_The condensation is induced in the vicinity of the laser beam typically from 25 s to 150 s after the beghming of illumination, and the nucleation extends to th.e whole chamber 3 to 5 min later. When the Iaser beam is turned off the condensation stops slowly 5 to fO min later. We have plotted in f?gs_ 1-3 the variation of the critical intensity (total intensity of the laser beam of I.8 mm diameter) with different parameters_ Fig_ l. gives the critical iight intensity as a function of the
f November 1978
LETTERS
\\ t
a.
\
\\
\
\
-o-y
k* !
1
2
I
3
I
4
Fig. t . Plot of the criticaliight inter&v as a frrnctionof supersaturation for two wavetengths The pressare of &Iorine is in the rage of 5 to LO ton and the tempemttxre between 276 and 281 K. The vertical bars represent the unceminties in the estimation of the ixghning of condensatioa.
supersaturation ratio S of CC4 with a total pressure of 1400 torr, the partial pressure of Cl2 being in the range of 5 to 10 tom Experiments performed close to the critical supersaturation S, show that the critical intensity to initiate photonucleation becomes very low in this range of supersaturationThe phenOMenOn is f&y web reproducible: at a given supersaturation, Cl2 partial pressure arzd temperature, the critical intensity or threshold remains constant_ In fig. 2 we have plotted the variation of the critical intensity as a function of wavelength, and in fig. 3 its variation with the partial pressure of Cl2. The photonucleation phenomenon obsenred is directly connected to the absorption spectrum of chlorine which has for the wavelengths of the argon ion taser a continuous and weak absorption band; it corresponds to the dissociation of CI 2 molecules into two atoms IfS,f6] _ A different experiment has been performed in tie
Volume
59,number1
CHEMICAL
1 November 1978
PHYSICS LETTERS
following conditions: operating of the chamber at a supersaturation of 2.6,a partial pressure of Cl2 of 8 torr, an illumination for a short period (I= = 30 mW at A) and then illumination for 170 min with 1 W intensity_ In such conditions, when repeating the experiment 12 h later, the critical intensity measured on the same vapor had enhanced roughly by a factor of ten.
3. Concfusion We think that our results demonstrate the nucleating effect of visible light. This phenomenon is very lmuch sensitive to the wavelength and the intensity of light; it thus confirms similar results obtained on different systems but in the presence of UV radiation ]4.5] _ Although we have not yet an eiaborate explanation for this photonucleation
phenomenon
one can
formulate two hypotheses. Remembering that we are deahng with a homogeneous binary nucleation process (Cl2--CCL+, He playing no role) [17] the free Fig. 2_ Plot of the critical intensity as a function of -velength at three supersaturations. The continuouscurvesare derivedfrom the contim~ousabsorptiofi of ch!oiiine [ 16]_
energy for the nucleation of a droplet with a molecules of CC& and b molecules of Cl2 is given by: 4G = --ukT ln (Sl fN1) - bkT In (S&5)
+ (uq + b02)~/~(36#
(1)
where S1 and .S2 are the supersaturation ratios of CC4 and Cl2, u1 and u2 the molar volumes, N1 and
Ic mw ld
30,
i sz2.6
iV2 being their mole fractions in the droplet. o is the surface tension. One must firstly think that the following reactions take place [IS] :
cl2 f hv + 2cl-,
2CC13 + C2Chj _ The formation of C2Cl6 (and C2C&) by photochemical reaction could thus increase S1 or S2 and produce the nucleation. Secondly one might thinkthat the radicals CC13 and atomic Cl form complexes NS with Ccl, Fig. 3. Plot of the critical Iight intensity as a function of partial pressure of chlorine in the Iaser beam. The wave length is 4880 A.
which
are in an excited state (CC13 f CC4 + N*). These complexes are nuclei which might initiate the condensation phenomenon. Following Makhviladze’s [2] treatment one can understand 115
Volume 59. number
1
CHFMICAL
PHYSICS
that in presence of photoexcited molecules AG, eq_ (l), must be modiiied to include a suppIementary term, which is a function of the light intensity, which accounts for the presence of excited states in the vapor (the chemicai potential of N* being different from that of the non-excited CC14 moiecuks). This would decrease the free energy of nucleation and be in favor of condensation. After a long and intensive light exposure, nucIei remain in the diffusion chamber and CC13 and Cl can de-excite by colhsion with these nuclei; this couid explain the increase of the critical intensity in the last series of experiments which we reported. Seems ra The first mechanism. C2a6 fOmlatiOn, be the Iess effective nucleation initiator. Further experiments are in progress (in particular chromatographic analysis) to elucidate the complete mechanism for the photonucIeation which invoIves certainIy a chain ofphotachemical reactions-
LETFJZRS
VI J-L Katz, CJ- ScoppaJJ.N-G_Kumar
LL Gudzenko. T-M. ~f&hvibdze and M-E. Sa~~ckev, Phys_ Letters 56A (1976) 289_ 121 T.&f. Makhhdze and ME Sarychev, Soviet Phys JETP 44 (1977) 834131 B_A_ Beayglij. E-F. Gab&in and J.Ia. Dudin, Soviet Phys- JETP Letters 22 (1975) 76.
III
1978
and P. Xirabel,
Chem.Phys-62 (1975) 448. PI J-L Katz. F-C. Wen, T. McLaughlin, RJ. Rench and J.
WI [71 181 191
WI 1111
WI 1131
1141
References
1 November
1151 1161 [I71
1W
R. Pa&h, Sccce 200 (i978) 769. P.M. McMurr and SK. Friedlander, J. CoBoid Interface ScL 64 (1978) 248. K_ I-oto. pi_ Presser and I. Ross, J. Chem. Phyr 68 (1978) 663. 19. Herman, R.P_ Marie&x Jr_ and A. Javan. J. Chem. Phys- 68 (1978) 1070_ RV- Ambartzumian, Y_A_ Gorokhov. V_S_ Letokhov, G.M. hfakarov and A.A. Puretzki, Phys. Letters 56A (1976) 183. V- Skak, J. Caballero, A. Burgos and E. Quel, Chem. Phys Letters 54 (1978) 205. R-E. Huie, J-T. Herron, W. Braun and W_ Tsang, Chem. Phys Letters 56 (1978) 193. J-I, Katz and BJ. Qstermeir, J. Chem. Phys 47 (1967) 478. F. Barbut, M&ode d*AnaJyse par Diffusion de la Lumi&e et Cx&lation de Photons de k NucGation dans une Vapeur Sursatu&e, Th&se, Paris Ao 12192 (1975). J-L. Katz and P- Mirabel. J. Chem. Phys. 67 (1977) 1697. N.S. Bay&s, Trans Faraday Sot. 33 (1937) 1339. R-G. Aickin and N.S. Bay&s, Trans. Faraday Sot. 33 (1937) 1333. R-A_ Sigsbee. in: Nuckation, ed. AC. ZettJemoyer (De&her, New York, 1969). N. Davidson and J-H. Sullivan, J. Chem. Phys- 17 (1949) 176.
1
CHEMICAL
PHYSICS
LE’ITERS
1 November 1978
PHOTONUCLEATION IN A SUPERSATURATED VAPOR B. CORDIER and P_ PAPON Luboratoire de Rkonance Magnefique. Equipe de Recherche associ&eau C.N.R.S.. Ecole Suphieure de PI2ysique et de CWmie IndustrieIIes. 75005 PZU& France Received 10 July 1978 We report experimental results of the investigation of the photonuckation process in a diffusion chamber. We have shovm that nucleation can be induced in a supersaturated vapor of Ca in the presence of a small concentration of C!* at A = 4880 A, 5145 A using laser light. The process is strong& dependent upon the intensity and wavelength of light and upon the supersturation of the vapor.
1. Iutroduction Recent experiients performed in various conditions have revealed the possibility of inducing the gas-livid transition by light. Eventual photonucleation initiated in a vapor by a resonant radiation has been widely discussed by Gudzenko [I] and by Mlikhviladze and Sarychev [2] _ They showed that radiation which is absorbed by a vapor can change the pair interaction potentials between the moiecules and thus the parameters of the equation of state for the gas-liquid system. The coexistence curve and the spinodal might be shifted under illumination and a supersaturated vapor might thus nucleate_ Their calculations seem to account for a photonucleation phenomenon observed in iodine [3]_ Katz axedCo1 [4] have observed that on irradiation with light of wavelength shorter than 420 nm certain organic compounds, even at very low concentration, are able to cause the nucleation of supersaturated vapors. They interpreted their results with a mechanism in which nuclei with only a few photoexcited molecules were able to initiate the nucleation process within the vapor. The same authors have recently reported similar experiments on the photoinduced nucleation of water vapor [S] and they have shown that it is an intrinsic property of water, the phenomenon occurring even in the absence of an impurity. Many other experiments have been performed in the field of photochemically generated aerosols, they
have particularly shown that ultraviolet light can produce aerosols in vapor systems, the chemical reaction being induced by moiecules or radicals in excited states [6,7]. Chemical reactions in the gas phase can also be stimulated by laser light following the same mechanisms as reported for example by I-Ierman and Co1 IS] _ Finally, isotope-selective dissociation of molecules by laser radiation in the infrared spectrum have tended to be extensively studied in various systems as, for example, CC14 and CF,Cl? 19-l 1] . Isotopic enrichment in some atomic species is one of the motivations behind such studies. We wish to report preliminary results of an investigation of nucleation of supersaturated CC4 induced by light in the visible part of the spectrum.
2. Experiment21 results We studied the nucleation phenomenon in a supersaturated vapor with a thermal diffusion cloud chamber. This chamber 1121 can maintain a vapor in a state of continuous supersaturation and consists of two horizontal and circular copper plates of 520 mm diameter separated by a stainless steel ring 50 mm high and 450 mm of inner diameter with four glass windows- The lower plate is maintained at a high temperature and the upper plate at a low temperature; one-dimensional heat and mass flows settle between the two plates. A computer solution of the 113
vobmle
SF_ number I
CHEitfKXL
PHYSICS
heat conduction and mass diffusion equations gives the temperature and partial pressure profles within the vapor_ CC4 is introduced in the chamber with a hght carrier gas as helium. With the experimental device we studied first the vapor system by Raylcigh light scattering using an argon ion laser as a light source. Preliminary measurements performed by Barbut [13] showed an anornalousIy Iarge intensity of light scattered by the vapor at iow supersaturation ratio- Carefui investigation revealed that the faser be&n was inducing nucleation. A thorough examination of the phenomenon has since revealed that the photonucieation in CC4 was due to the presence of impurities wi?ich were proved to be Ciz. We have shown that pure CC14 in presence of light does not nucleate until the supersaturation reaches its critical value; conversely one can induce nucleation by light in CC14 in the presence of Ci2 at low concentration even at iow supersaturation. We have thoroughly investigated this photonucleation phenomenon in the Ci&-cg2-He vapor system. The heat and mass transfer equations have been modified to take into account the presence of Ciz fI4] and the pressure and temperature profdes recalculated_ The copper plates have been protected against corrosion. Experiments have been performed at various frequencies @ = 4880 & 5 14.5A _ . _) and at tiifferent light intensities_ An experiment performed with an He-Ne Iaser (X = 6328 L%,35 mW) showed LO effect of ii& on nucleation at various supersaturation ratios. We chose to measure a parameter which we clrll the critical intensity_ It represents the light intensity which is needed to produce a nuckation rate of one t0 tW0 drO2.S CM-’ S- ’ in two minutes (the time interval is chosen constant for all experiments) following iJiurnination_The condensation is induced in the vicinity of the laser beam typically from 25 s to 150 s after the beghming of illumination, and the nucleation extends to th.e whole chamber 3 to 5 min later. When the Iaser beam is turned off the condensation stops slowly 5 to fO min later. We have plotted in f?gs_ 1-3 the variation of the critical intensity (total intensity of the laser beam of I.8 mm diameter) with different parameters_ Fig_ l. gives the critical iight intensity as a function of the
f November 1978
LETTERS
\\ t
a.
\
\\
\
\
-o-y
k* !
1
2
I
3
I
4
Fig. t . Plot of the criticaliight inter&v as a frrnctionof supersaturation for two wavetengths The pressare of &Iorine is in the rage of 5 to LO ton and the tempemttxre between 276 and 281 K. The vertical bars represent the unceminties in the estimation of the ixghning of condensatioa.
supersaturation ratio S of CC4 with a total pressure of 1400 torr, the partial pressure of Cl2 being in the range of 5 to 10 tom Experiments performed close to the critical supersaturation S, show that the critical intensity to initiate photonucleation becomes very low in this range of supersaturationThe phenOMenOn is f&y web reproducible: at a given supersaturation, Cl2 partial pressure arzd temperature, the critical intensity or threshold remains constant_ In fig. 2 we have plotted the variation of the critical intensity as a function of wavelength, and in fig. 3 its variation with the partial pressure of Cl2. The photonucleation phenomenon obsenred is directly connected to the absorption spectrum of chlorine which has for the wavelengths of the argon ion taser a continuous and weak absorption band; it corresponds to the dissociation of CI 2 molecules into two atoms IfS,f6] _ A different experiment has been performed in tie
Volume
59,number1
CHEMICAL
1 November 1978
PHYSICS LETTERS
following conditions: operating of the chamber at a supersaturation of 2.6,a partial pressure of Cl2 of 8 torr, an illumination for a short period (I= = 30 mW at A) and then illumination for 170 min with 1 W intensity_ In such conditions, when repeating the experiment 12 h later, the critical intensity measured on the same vapor had enhanced roughly by a factor of ten.
3. Concfusion We think that our results demonstrate the nucleating effect of visible light. This phenomenon is very lmuch sensitive to the wavelength and the intensity of light; it thus confirms similar results obtained on different systems but in the presence of UV radiation ]4.5] _ Although we have not yet an eiaborate explanation for this photonucleation
phenomenon
one can
formulate two hypotheses. Remembering that we are deahng with a homogeneous binary nucleation process (Cl2--CCL+, He playing no role) [17] the free Fig. 2_ Plot of the critical intensity as a function of -velength at three supersaturations. The continuouscurvesare derivedfrom the contim~ousabsorptiofi of ch!oiiine [ 16]_
energy for the nucleation of a droplet with a molecules of CC& and b molecules of Cl2 is given by: 4G = --ukT ln (Sl fN1) - bkT In (S&5)
+ (uq + b02)~/~(36#
(1)
where S1 and .S2 are the supersaturation ratios of CC4 and Cl2, u1 and u2 the molar volumes, N1 and
Ic mw ld
30,
i sz2.6
iV2 being their mole fractions in the droplet. o is the surface tension. One must firstly think that the following reactions take place [IS] :
cl2 f hv + 2cl-,
2CC13 + C2Chj _ The formation of C2Cl6 (and C2C&) by photochemical reaction could thus increase S1 or S2 and produce the nucleation. Secondly one might thinkthat the radicals CC13 and atomic Cl form complexes NS with Ccl, Fig. 3. Plot of the critical Iight intensity as a function of partial pressure of chlorine in the Iaser beam. The wave length is 4880 A.
which
are in an excited state (CC13 f CC4 + N*). These complexes are nuclei which might initiate the condensation phenomenon. Following Makhviladze’s [2] treatment one can understand 115
Volume 59. number
1
CHFMICAL
PHYSICS
that in presence of photoexcited molecules AG, eq_ (l), must be modiiied to include a suppIementary term, which is a function of the light intensity, which accounts for the presence of excited states in the vapor (the chemicai potential of N* being different from that of the non-excited CC14 moiecuks). This would decrease the free energy of nucleation and be in favor of condensation. After a long and intensive light exposure, nucIei remain in the diffusion chamber and CC13 and Cl can de-excite by colhsion with these nuclei; this couid explain the increase of the critical intensity in the last series of experiments which we reported. Seems ra The first mechanism. C2a6 fOmlatiOn, be the Iess effective nucleation initiator. Further experiments are in progress (in particular chromatographic analysis) to elucidate the complete mechanism for the photonucIeation which invoIves certainIy a chain ofphotachemical reactions-
LETFJZRS
VI J-L Katz, CJ- ScoppaJJ.N-G_Kumar
LL Gudzenko. T-M. ~f&hvibdze and M-E. Sa~~ckev, Phys_ Letters 56A (1976) 289_ 121 T.&f. Makhhdze and ME Sarychev, Soviet Phys JETP 44 (1977) 834131 B_A_ Beayglij. E-F. Gab&in and J.Ia. Dudin, Soviet Phys- JETP Letters 22 (1975) 76.
III
1978
and P. Xirabel,
Chem.Phys-62 (1975) 448. PI J-L Katz. F-C. Wen, T. McLaughlin, RJ. Rench and J.
WI [71 181 191
WI 1111
WI 1131
1141
References
1 November
1151 1161 [I71
1W
R. Pa&h, Sccce 200 (i978) 769. P.M. McMurr and SK. Friedlander, J. CoBoid Interface ScL 64 (1978) 248. K_ I-oto. pi_ Presser and I. Ross, J. Chem. Phyr 68 (1978) 663. 19. Herman, R.P_ Marie&x Jr_ and A. Javan. J. Chem. Phys- 68 (1978) 1070_ RV- Ambartzumian, Y_A_ Gorokhov. V_S_ Letokhov, G.M. hfakarov and A.A. Puretzki, Phys. Letters 56A (1976) 183. V- Skak, J. Caballero, A. Burgos and E. Quel, Chem. Phys Letters 54 (1978) 205. R-E. Huie, J-T. Herron, W. Braun and W_ Tsang, Chem. Phys Letters 56 (1978) 193. J-I, Katz and BJ. Qstermeir, J. Chem. Phys 47 (1967) 478. F. Barbut, M&ode d*AnaJyse par Diffusion de la Lumi&e et Cx&lation de Photons de k NucGation dans une Vapeur Sursatu&e, Th&se, Paris Ao 12192 (1975). J-L. Katz and P- Mirabel. J. Chem. Phys. 67 (1977) 1697. N.S. Bay&s, Trans Faraday Sot. 33 (1937) 1339. R-G. Aickin and N.S. Bay&s, Trans. Faraday Sot. 33 (1937) 1333. R-A_ Sigsbee. in: Nuckation, ed. AC. ZettJemoyer (De&her, New York, 1969). N. Davidson and J-H. Sullivan, J. Chem. Phys- 17 (1949) 176.