- Email: [email protected]
Multiphoton excitation and dissociation of ethylene by intense 10 μm picosecond pulses
ELSEVIER
Journal of MOLECULAR STRUCTURE Journal of Molecular Structure 349 (1995) 219-222
Multiphoton excitation and dissociation picosecond pulses V.M.Gordienko, E.O.Danilov, V.T.Platonenko,
of ethylene by intense 10 pm
and V.A.Slobodyanyuk
Physics Department, International Laser Center, Moscow State University, II9899,Russia
1. INTRODUCTION Studies of multiphoton molecular resonant absorption (MPA) processes induced by intense 10 pm picosecond infrared laser radiation provide a successful method to investigate the dynamics of nonlinear molecular excitation and laser-controlled photophysical and photochemical reactions. A strong laser field of the 10 p,m picosecond pulses at the laser intensity of 1010 W/cm2 and their corresponding spectral bandwidth help to compensate molecular anharmonic energy mismatch and overcome the “bottleneck” effect at the low-lying levels. As a result of mixing of the electron states in the strong laser field the electronically excited intermediates can appear as products of IR laser photolysis[l]. Chemical reactions more expected by UV photolysis can occur between the initial molecules and the products due to their high chemical activity, and more complicated molecules appear after the irradiation. In this paper we present the results on the optoacoustic measurements of the absorbed energy in ethylene, observation of visible luminescence of electronically excited C2 radicals and detection of products of the dissociation by the gas-chromatography. 2. EXPERIMENTAL The experiments were performed with a picosecond-pulse CO2 system described in detail in [2]. It was operating in three different regimes. In the first regime a seed picosecond 10 pm pulse is produced in a double successive difference frequency generation scheme, pumped by a solid-state Nd:YAl03 picosecond laser (X=1.08 pm). The duration of the pulse is 9 psec; the energy of this pulse is 1 ClJ.The high-pressure TE CO2 module amplifies this weak picosecond 10 pm pulse up to mJ energies in a regenerative regime. The output pulse (75 ns FWHM) consists of a train of short pulses separated by 15 ns. The duration of the short pulse is 6 psec, maximum pulse energy is about 2 mJ, the total energy of the train is about 15 mJ. For some multiphoton excitation experiments we used the high-pressure TE CO2 module as a “broadband ” nanosecond oscillator with pulse duration of 75 nsec FWHM and output energy of 20 mJ. The laser bandwidth was measured to be of about 1.5 cm-l, the wavelength was adjusted by intracavity NaCl prism. The third operating regime was a tunable injection locking of the high-pressure TE CO2 module to produce nanosecond pulses with “narrow” (about 10-3 cm-l FWHM) spectrum (pulse duration of 75 nsec, energy of 20 mJ). Experiments on the multiphoton absorption and dissociation in C2H4 using the train of the 0022-2860/95/%09.50 0 1995 Elsevier Science B.V. SSDI 0022-2860(95)08748-6
All rights reserved
220
picosecond pulses, “broadband”, and “narrow” nanosecond pulses were performed. To study the multiphoton excitation of C2H4 molecules we used technique of optoacoustic absorption measurements. The output radiation was focused by means of a NaCl lens (focal length of 73 mm) into a 45 cm long gas cell with NaCl windows containing C2H4 at a pressure of 5 torr. A capasitor optoacoustic microphone was placed 5 cm behind the input window near the lens’s focus lcm above the beam waist. Buffer volumes and apertures were put into gas cell aiming to extract the optoacoustic signal only from the focal area of about 5 mm long and suppress the background signal due to the absorption off the central part of the caustic area[3]. The beam profile measured in a focal plane of the lens had a Gaussian form with l/eVevel diameter of about 120 pm. So the maximum fluence at the focus was about 100 J/cm2 for the train of the picosecond pulses and above 100 J/cm2 for both “broadband” and “narrow” nanosecond pulses. On the other hand, the maximum intensity at the focus of the lens appeared to be about 2 109 W/cm2 for “broadband” and “narrow” nanosecond pulses while it reached 1012 W/cm2 for the picosecond pulses. A visible luminescence resulting from excitation over a dissociation limit was registered from region of the lens focus and was wiewed through a side-window of the gas cell by a photomultiplier (PMT) simultaneously with the absorbed energy measurements. An interference filter put before the PMT enabled isolation of a spectral feature at 5 16 nm due to electronically excited C2 radicals. A pure 99.99% ethylene was used in the experiments without any additional purification. Both initial gas and the gas after series of 10, 60, and 200 laser shots was analized by gaschromatography analizer to identity a stable products of the dissociation. 3. RESULTS AND DISCUSSION Fig. 1 shows dependencies of the average absorbed energy in quanta per molecule on the laser fluence in the beam waist (log-log scale) for C2H4 irradiated by a train of the 10 urn picosecond pulses, “broadband” nanosecond pulses, and by “narrow” spectrum nanosecond pulses near 949 cm-l. As it is seen, a straight lines (1,2,3) can be drawn through the data on the log-log plane in the laser fluence region up to above 100 J/cm2. The average absorbed energy expressed in quanta per molecule after the careful calibration of the optoacoustic detector appears to exceed the dissociation limit of 40 quant/molec reported previously [3] for both picosecond and “broadband” nanosecond pulses. The former line crosses the dissociation threshold at 20 J/cm2, the latter at 45 J/cm2, that is ensured by an appearance of the luminiscence signal at these fluences. Both of the lines show no saturation above the dissociation limit. The line corresponding to the “narrow”-band nanosecond pulses did not reach the dissociation barrier. The difference in the absorption between the “broadband” and “narrow’‘-band nanosecond radiation was discussed in [4] to explain the results of multiphoton excitation of SF6 molecules by picosecond pulses. The growth of the absorption with the laser bandwidth can be caused by presence of groups of states in a quasicontiinum with rather different dipole moments and detunings from resonance. Such “nondense” zones demonstrate a dependence on a laser bandwidth until it become compared with the spectral interval between adjacent zones. Our results show that the absorption is sensitive to laser intensity as well. The broadening of molecular lines caused by pulse saturation of molecular quantum states in a strong field for C2H4 can reach of about 10 cm-l at I=1 011 W/cmz. The laser intensity acts like the laser
< II>,
1
10
quant
/mo 1
I
100
arb.
I”..
0.01 F.
J /cm2
Fig.1. Dependence of average absorbed energy on the aser fluence. P(C2H4)=5 Torr. 1 is the train of the Gcosecond pulses (near 949 cmW1), 2 is “broadband” nanosecond pulses (r- near 949.5 cm-‘, o - near 948 cm-‘) 3,4 are “narrow” band nanosecond pulses (at 949.48 cm“ md 947.74 cm-t).
10
“n
,
F.
J/cm’
100
Fig.2. Dependence of the luminescence signal on the laser fluence. 1 is the train of the picosecond pulses (near 949 cm-‘), 2 iz “broadband” nanosecond pulses (near 949 cm-‘).
spectrum broadening, allowing to overcome the molecular anharmonicity and enhances the absorption. Thus, the train of the picosecond pulses and the “broadband” nanosecond pulses excite C2H4 molecules more effectively than nanosecond pulses with “narrow” spectrum, providing a correspondingly different excitation rates for these regimes of interaction. Fig.2 shows a dependence of luminescence signal on the laser fluence in the beam waist. The signal grows rapidly as the laser fluence increases. The luminescence signal appeared between 20 and 30 J/cm2 for the picosecond pulses, and at 45 J/cm2 for the “broadband’ nanoscond pulses in a good agreement with the optoacoustic measurements. There was no luminescence signal in case of the “narrow” band nanosecond pulses. Approaching the dissociation threshold the excitation rate should be compared with the dissosiation rate, controlling the overheating degree, and channels of decomposition. A processes of laser photochemistry of ethylene should be taken into account to explain first - the absence of a saturation of the OA sigma1 above the dissociation limit, and, second - formation of the C2-radicals in electronically excited Cu state. It is believed now, that C2H4 decomposes to form acethylene C2H2, ethane C2&, and lpenthene C5Hs as stable products by a collisionless IR laser photolys[3]. In our experiments the only acethylene was detected after irradiation of the ethylene by 10 shots in the picosecond and “broadband” nanosecond regime.and no traces of methane was found. As the number of laser shots increases, more complicated unidentified products appear. The average conversion by irradiation of the nanosecond pulses is about a half of that for picosecond pulses. But, in case of 200 shots of the nanosecond pulses up to 60% of the converted ethylene consists of rather complicated unidentified molecules, while that amount for 200 shots of the picosecond pulses does not exceed 5%. At the same time the percentage of acethylene for the picosecond pulses is 4-5 times of that for the nanosecond pulses. From up to date understanding of a mechanism of a pyrolisis of the ethylene the most likely channel of the monomolecular decomposition seems to be the formation of acethylene by the
222
concert mechanism with the energy threshold of the reaction of about 53 KcaVmol (18 quant/molec). But, in our experiments quite different mechanism could play which is proved by simultaneous appearance of the dissociation products and the luminescence signal at higher threshold. We suppose that the high intensity of picosecond IR pulses and correspondingly high rate of the vibrational excitation could result in the decomposition through another channels which are more featurable for experiments on UV laser photolysis of C$-I4 [5], when short-living intermediates can appear due to decay from the electronically excited states of the ethylene to form vinylidene: CH2=CH2 ->{nhv(IR)}->CHz=CHz* --> CHz=C:* as a first stage. Than the vinylidene can absorb the laser radiation itself since it has a vibrational mode shifted by 20 cm-l from initial ~7 vibration of ethylene [6] that could be overlapped by Stark broadening at the high IR laser intensity giving rise to formation of electronically excited C2 radicals: CHz=C:*--> {nhv(IR)} --> C2*, or, if it does not absorb, the isomerization reaction HHC=C:* -> HC=CH* could occur giving rise to formation of acethylene. The unstable intermediates such as vinylidene or C2* radicals having very high chemical activity can react with initial molecules or with stable products to form more complicated secondary products. In case of exothermic character of these reactions the heat extracted can affect the optoacoustic signal leading to absence of the saturation. Under some experimental conditions the heat of the reaction can be measured in this way. 4. ACKNOWLEDGEMENT The authors acknowledge Drs. Yu.N.Zhitnev, V.V.Timofeyev, and N.Yu.Ignatieva of Chemistry Department of Moscow State University for helpful discussions. REFERENCES 1. R.Grunvald, et al, Khimicheskaya fizika (in Russian), 4, (1985) 46. 2. V.M.Gordienko,et al, Proc. SPIE vol 204 1, (1994) 193. 3. V.S.Letokhov, “Nonlinear Selective Photoprocesses in atoms and molecules” (in Russian), Moscow, 1983. 4. S.S.Alimpiev, et al, Appl.Phys.B, 35, (1984) 1. 5. J.R.McNesby, et al, J.Chem.Phys, 36, (1962) 601. 6. Y.Yamaguchi. et al. J.Chem.Phvs. 100.(1994). 4969.
Journal of MOLECULAR STRUCTURE Journal of Molecular Structure 349 (1995) 219-222
Multiphoton excitation and dissociation picosecond pulses V.M.Gordienko, E.O.Danilov, V.T.Platonenko,
of ethylene by intense 10 pm
and V.A.Slobodyanyuk
Physics Department, International Laser Center, Moscow State University, II9899,Russia
1. INTRODUCTION Studies of multiphoton molecular resonant absorption (MPA) processes induced by intense 10 pm picosecond infrared laser radiation provide a successful method to investigate the dynamics of nonlinear molecular excitation and laser-controlled photophysical and photochemical reactions. A strong laser field of the 10 p,m picosecond pulses at the laser intensity of 1010 W/cm2 and their corresponding spectral bandwidth help to compensate molecular anharmonic energy mismatch and overcome the “bottleneck” effect at the low-lying levels. As a result of mixing of the electron states in the strong laser field the electronically excited intermediates can appear as products of IR laser photolysis[l]. Chemical reactions more expected by UV photolysis can occur between the initial molecules and the products due to their high chemical activity, and more complicated molecules appear after the irradiation. In this paper we present the results on the optoacoustic measurements of the absorbed energy in ethylene, observation of visible luminescence of electronically excited C2 radicals and detection of products of the dissociation by the gas-chromatography. 2. EXPERIMENTAL The experiments were performed with a picosecond-pulse CO2 system described in detail in [2]. It was operating in three different regimes. In the first regime a seed picosecond 10 pm pulse is produced in a double successive difference frequency generation scheme, pumped by a solid-state Nd:YAl03 picosecond laser (X=1.08 pm). The duration of the pulse is 9 psec; the energy of this pulse is 1 ClJ.The high-pressure TE CO2 module amplifies this weak picosecond 10 pm pulse up to mJ energies in a regenerative regime. The output pulse (75 ns FWHM) consists of a train of short pulses separated by 15 ns. The duration of the short pulse is 6 psec, maximum pulse energy is about 2 mJ, the total energy of the train is about 15 mJ. For some multiphoton excitation experiments we used the high-pressure TE CO2 module as a “broadband ” nanosecond oscillator with pulse duration of 75 nsec FWHM and output energy of 20 mJ. The laser bandwidth was measured to be of about 1.5 cm-l, the wavelength was adjusted by intracavity NaCl prism. The third operating regime was a tunable injection locking of the high-pressure TE CO2 module to produce nanosecond pulses with “narrow” (about 10-3 cm-l FWHM) spectrum (pulse duration of 75 nsec, energy of 20 mJ). Experiments on the multiphoton absorption and dissociation in C2H4 using the train of the 0022-2860/95/%09.50 0 1995 Elsevier Science B.V. SSDI 0022-2860(95)08748-6
All rights reserved
220
picosecond pulses, “broadband”, and “narrow” nanosecond pulses were performed. To study the multiphoton excitation of C2H4 molecules we used technique of optoacoustic absorption measurements. The output radiation was focused by means of a NaCl lens (focal length of 73 mm) into a 45 cm long gas cell with NaCl windows containing C2H4 at a pressure of 5 torr. A capasitor optoacoustic microphone was placed 5 cm behind the input window near the lens’s focus lcm above the beam waist. Buffer volumes and apertures were put into gas cell aiming to extract the optoacoustic signal only from the focal area of about 5 mm long and suppress the background signal due to the absorption off the central part of the caustic area[3]. The beam profile measured in a focal plane of the lens had a Gaussian form with l/eVevel diameter of about 120 pm. So the maximum fluence at the focus was about 100 J/cm2 for the train of the picosecond pulses and above 100 J/cm2 for both “broadband” and “narrow” nanosecond pulses. On the other hand, the maximum intensity at the focus of the lens appeared to be about 2 109 W/cm2 for “broadband” and “narrow” nanosecond pulses while it reached 1012 W/cm2 for the picosecond pulses. A visible luminescence resulting from excitation over a dissociation limit was registered from region of the lens focus and was wiewed through a side-window of the gas cell by a photomultiplier (PMT) simultaneously with the absorbed energy measurements. An interference filter put before the PMT enabled isolation of a spectral feature at 5 16 nm due to electronically excited C2 radicals. A pure 99.99% ethylene was used in the experiments without any additional purification. Both initial gas and the gas after series of 10, 60, and 200 laser shots was analized by gaschromatography analizer to identity a stable products of the dissociation. 3. RESULTS AND DISCUSSION Fig. 1 shows dependencies of the average absorbed energy
< II>,
1
10
quant
/mo 1
I
100
arb.
I”..
0.01 F.
J /cm2
Fig.1. Dependence of average absorbed energy
10
“n
,
F.
J/cm’
100
Fig.2. Dependence of the luminescence signal on the laser fluence. 1 is the train of the picosecond pulses (near 949 cm-‘), 2 iz “broadband” nanosecond pulses (near 949 cm-‘).
spectrum broadening, allowing to overcome the molecular anharmonicity and enhances the absorption. Thus, the train of the picosecond pulses and the “broadband” nanosecond pulses excite C2H4 molecules more effectively than nanosecond pulses with “narrow” spectrum, providing a correspondingly different excitation rates for these regimes of interaction. Fig.2 shows a dependence of luminescence signal on the laser fluence in the beam waist. The signal grows rapidly as the laser fluence increases. The luminescence signal appeared between 20 and 30 J/cm2 for the picosecond pulses, and at 45 J/cm2 for the “broadband’ nanoscond pulses in a good agreement with the optoacoustic measurements. There was no luminescence signal in case of the “narrow” band nanosecond pulses. Approaching the dissociation threshold the excitation rate should be compared with the dissosiation rate, controlling the overheating degree, and channels of decomposition. A processes of laser photochemistry of ethylene should be taken into account to explain first - the absence of a saturation of the OA sigma1 above the dissociation limit, and, second - formation of the C2-radicals in electronically excited Cu state. It is believed now, that C2H4 decomposes to form acethylene C2H2, ethane C2&, and lpenthene C5Hs as stable products by a collisionless IR laser photolys[3]. In our experiments the only acethylene was detected after irradiation of the ethylene by 10 shots in the picosecond and “broadband” nanosecond regime.and no traces of methane was found. As the number of laser shots increases, more complicated unidentified products appear. The average conversion by irradiation of the nanosecond pulses is about a half of that for picosecond pulses. But, in case of 200 shots of the nanosecond pulses up to 60% of the converted ethylene consists of rather complicated unidentified molecules, while that amount for 200 shots of the picosecond pulses does not exceed 5%. At the same time the percentage of acethylene for the picosecond pulses is 4-5 times of that for the nanosecond pulses. From up to date understanding of a mechanism of a pyrolisis of the ethylene the most likely channel of the monomolecular decomposition seems to be the formation of acethylene by the
222
concert mechanism with the energy threshold of the reaction of about 53 KcaVmol (18 quant/molec). But, in our experiments quite different mechanism could play which is proved by simultaneous appearance of the dissociation products and the luminescence signal at higher threshold. We suppose that the high intensity of picosecond IR pulses and correspondingly high rate of the vibrational excitation could result in the decomposition through another channels which are more featurable for experiments on UV laser photolysis of C$-I4 [5], when short-living intermediates can appear due to decay from the electronically excited states of the ethylene to form vinylidene: CH2=CH2 ->{nhv(IR)}->CHz=CHz* --> CHz=C:* as a first stage. Than the vinylidene can absorb the laser radiation itself since it has a vibrational mode shifted by 20 cm-l from initial ~7 vibration of ethylene [6] that could be overlapped by Stark broadening at the high IR laser intensity giving rise to formation of electronically excited C2 radicals: CHz=C:*--> {nhv(IR)} --> C2*, or, if it does not absorb, the isomerization reaction HHC=C:* -> HC=CH* could occur giving rise to formation of acethylene. The unstable intermediates such as vinylidene or C2* radicals having very high chemical activity can react with initial molecules or with stable products to form more complicated secondary products. In case of exothermic character of these reactions the heat extracted can affect the optoacoustic signal leading to absence of the saturation. Under some experimental conditions the heat of the reaction can be measured in this way. 4. ACKNOWLEDGEMENT The authors acknowledge Drs. Yu.N.Zhitnev, V.V.Timofeyev, and N.Yu.Ignatieva of Chemistry Department of Moscow State University for helpful discussions. REFERENCES 1. R.Grunvald, et al, Khimicheskaya fizika (in Russian), 4, (1985) 46. 2. V.M.Gordienko,et al, Proc. SPIE vol 204 1, (1994) 193. 3. V.S.Letokhov, “Nonlinear Selective Photoprocesses in atoms and molecules” (in Russian), Moscow, 1983. 4. S.S.Alimpiev, et al, Appl.Phys.B, 35, (1984) 1. 5. J.R.McNesby, et al, J.Chem.Phys, 36, (1962) 601. 6. Y.Yamaguchi. et al. J.Chem.Phvs. 100.(1994). 4969.