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Photoionization studies of some small molecules
Journal of ElectronSpectroscopyand RelatedPhenomena79 (1996)483-485
Photoionization studies of some small molecules* Yunwu Zhang, Liusi Sheng, Fei Qi, Hui Gao, Shuqin Yu National Synchrotron Radiation Laboratory, University of Science & Technology of China, Hefei, 230029, China This paper reports the photoionization studies of C2H3C1 and CHCIF 2 performed at NSRL. A few Rydberg series of C2H3C1and some bond dissociation energies of C2H3CI and CHC1Fz will be given. 1. INTRODUCTION Photoionization of isolated molecules produces photoelectrons, phtoions or ionic and neutral fragments and even fluorescence photons. By detecting and analyzing these products one can obtain a number of important data for the molecules, such as state energies, absorption cross sections, ionization efficiencies, ionization potentials, dissociation energies, energy transfers, reaction channels and so on. Photoionization studies need an exciting light source which can provide photons in the range of 10-100eV for outer shell excitation and 100-1000eV for inner shell excitation of molecules. Synchrotron radiation meets these requirements very well. A photon beamline and an experimental station have been built at NSRL for these purposes. Photoionization studies of molecules can be done at the station by a few methods including molecular beam photoionization mass spectrometry, threshold photoelectron spectroscopy and threshold photoelectron-photoion coincidence, and useful results have been achieved for the last two years. In this paper we describe the photoionization of C2H3CI and CHCIFz using molecular beam photoionization mass spectrometry. 2. E X P E R I M E N T A L The experimental setup was described previously elsewhere, tll In brief, synchrotron radiation from the fifth bending magnet of the Hefei
800 MeV storage ring is focused by a toroidal mirror onto the entrance slit of a Im Seya-Namioka monochromator. The monochromator disperses the radiation in the range of 350-3000/~ with resolving power of greater than 500 with 220pm entrance and exit slits. The monochromatized radiation is focused by another toroidal mirror into the photoionization chamber. A lithium fluoride cutoff filter of 1 mm in thickness can be inserted into the beam path to eliminate second- and higher-order radiation when experiments are done at wavelengths longer than 1050A. The molecular beam is produced by expanding the sample gas, seeded in the carrier Ar or Ne gases through a 701am nozzle into the beam source chamber and later through a l mm skimmer into the photoionization chamber. It intersects the monochromatized synchrotron radiation beam at 70 mm from the nozzle. The photoionizaton chamber can be installed with two analyzers above and below the intersecting point. A experiment can be done with one or two of the prepared analyzers including a threshold photoelectron analyzer, a TOF and two quadrupole mass spectrometers and a photomultiplier depending on the requirements of the experiment. The photoionization and beam source chambers are pumped with a 1500 1/sec turbomolecular pump separately, the background pressure is 10 -4 Pa for the former and 102 Pa for the latter when the molecular beam is on. The setup is equipped a computerized control and data acquisition system. Experiments described here were performed only using one of the quadrupole mass spectrometers.
' This work was supported by the National Natural Science Foundation of China. 0368-2048/96/$15.00© 1996 ElsevierScienceB.V.All rights reserved PII S0368-2048 (96) 02900-3
484
The sample were C2H3CI of 99.9% purity and CHC1F2 of 99%. The pressure ratio of the samples to Ne was 1:5 to I:10 and the stagnation pressure was 1 to 3 bars. The monochromator was usually scanned with a wavelength increment of 0.20.5A and the data acquisition time for each point was I 0 to 20 seconds depending on ion abundance. 3. RESULTS
AND
",. ' : ":....;.
H+
Ca)
"..,"2' "
"
"~.",~. 665.7A
DISCUSSION
.......
3.1 Photoionization of C2H3C1 Fig.1 shows the PIE curve for C2H3C1+ from C2H3C1. The steep onsets at 9.98_+0.01eV and 11.66 + 0.05 eV are the ionization potentials corresponding to the ground and first excited electronic states of C2H3C1+. The energy difference 1.68 eV between these two states is in good agreement with our theoretical calculation value of 1.68 eV. The autoionization structure of the PIE curve in the range of 1060-1170A can be seen clearly. They were assigned as three Rydberg series ns, np and nd converging to the first excited state 2A' of C2H3C1+ and their quantum defects ~5(ns), 6(np) and 6(nd) were deduced to be 1.87, 1.51 and 0.22 respectively. 87
6
600
620
640
660
680
700
Wavelength/A
i,.',
C2H2C1÷
:.,,:,.;. (b)
'f.d" ,, 832.0A
7-'?,, . ::?'.? ....
""":"?:".',:-.~,.,.
7 3 0 .... 7~70~ ...... 8i(J . . . . . . . 850~ Wavelength/A
5
(c)
~ - " " ~ n s
C2H3 +
%,
% ~
600
690
780
870
988.9A
960 1050
Wavelength/A Fig. 2
illlllll~ 1056 ' ' '110() ' ' '1150
1200
PIE curves for H +, CzHzCI+ and CzH3+ from CzH3CI.
1250
Wavelength/A Fig. 1 PIE curve for C2H3C1÷ from C2H3C1 The appearance potentials of the fragment cations H +, CzH2C1+ and C2H3+ were measured to be 18.63+ 0.05 eV, 14.90+0.05 eV and 12.54+0.05 eV from their PIE curves ( shown in Fig. 2 (a, b, c) )
respectively. A few bond dissociation energies of C2H3C1 and C2H3C1÷ were evaluated from these data. The C-H bond dissociation energy Do(R-H) of a hydrocarbon can be determined by the following relation according to Field et al. [2] and Shiromaru et al.,[3] R-H + hv---~R + H++ e (1) Do(R-H)=AP(W)- IP(H) (2)
485 where the AP(H ÷) is the appearance potential of the fragment cation H ÷ from C2H3C1 and the ionization potential IP(H) of the H atom is well known as 13.598 eV, so we have Do(C2H~C1-H)=AP(H+)-IP(H)=5.03+0.05eV (3) The interference to the measurement of AP(H +) from the reaction channel R-H+hv-->R- + H + was neglected because we did not detect the C2HzCI in our experiment. The dissociation energies Do(CzHzCI+-H), Do(C2HzCI-H+) and Do(CzH3+-C1) of the molecular cation can be determined in the same way as above, Do(CzHzCI+-H)=AP(CzH2CI+)-IP(C2H3C1) =4.92+0.05 eV (4) Do(CzHzCI-H+)= AP(H+)-IP(CzH3CI) =8.65_+0.05 eV (5) Do(C2H3+_CI)=AP(C2H3+) - IP(C2H3C1) =2.56+0.05 eV (6) To determine the dissociation energy Do(CzH3-CI) the ionization potential of the radical C2H3 , IP(C2H3,), is needed. We failed to measure it. Li et al. t41 suggested that the most reasonable value of IP(CzH3,) should be 6.20 eV, so we have Do(C2H3-CI)-- AP(C2H3+) - IP(C2H3) --6.34+0.05 eV (7) The interference to the measurement of AP(C2H3+) from the channel C2H3CI + hv--> C2H3++ CI was omitted because we did not detect the ion CI. The above results are consistent with our theoretical calculation ones very well. I4]
4. S U M M A R Y
Photoionization studies of C2H3C1 and CHCIF 2 molecules were performed at NSRL by using the molecular beam photoionization mass spectrometry. A few Rydberg series of C2H3C1 were observed and some bond dissociation energies of C2H~CI, CHC1F 2 and their cations evaluated. .,
--
_
_ _
CHCIF2 +
(a)
. ...-..........~.. ~ •.
• • ..... "....."
~.
L"
12.16eV
940
960
980 1000 1020 Wacelength/~
1040
Cl+ i
. . . . .
•"v--.
(b) ""
i "..
I
'
18.51eV
I
'""-!..-..-..~--..-.-.-..-.....i 3.2 Photoionization of CHCIF2 PIEs for the formation of CHC1F2÷, C1+ from CHC[F 2 are displayed in Fig.3 (a, b). From these curves, the ionization potential for CHC1F2 and the appearance potential for C1+ can be observed: IP(CHC1F2)=12.16+ 0.02 eV and AP(CI+)=18.51+ 0.05eV. With these values, the dissociation energy, D0(CI+-CHF2)can be easily calculated, Do(CI+-CHF2)=AP(CI+)- IP(CHC1F2) =6.35+0.05eV (8) APs of CC1F2+, CHCIF +, CC1F÷, CHFz+, CF2+, CCF, CHCI +, HCI+, CF + and CHF + from CHC1F2 were also measured, their values are 14.19, 14.28, 15.00, 12.39, 18.79, 18.08, 19.87, 14.94, 15.90 and 18.50eV respectively. Some dissociation channels will be discussed by comparing with theoretical calculations in the future.
550
580
610 640 670 700 Wacelength/A Fig.3 PIEs of(a) CHCIF2 + and (b) CF from CHCIF 2. REFERENCES
I.Y.W. Zhang, L. S. Sheng, D. Q. Wang and T. Li, Chinese J. Chem. Phys. 5(5)(1992)321. 2 . F . H . Field and J. L. Franklin, ElEctron Impact
Phenomena and the Properties of Gaseous Ions, Academia Press, 1970, pp. 141-145. 3.H. Shiromaru, Y. Achiba, K. Kimura and Y. T. Lee, J. Phys. Chem. 91(1987)17. 4.L.S. Sheng, F. Qi, L. Tao, Y. W. Zhang, S. Q. Yu, C. K. Wong and W. K. Li, Int. J. Mass Spectrum. Ion Proc. (accepted).
Photoionization studies of some small molecules* Yunwu Zhang, Liusi Sheng, Fei Qi, Hui Gao, Shuqin Yu National Synchrotron Radiation Laboratory, University of Science & Technology of China, Hefei, 230029, China This paper reports the photoionization studies of C2H3C1 and CHCIF 2 performed at NSRL. A few Rydberg series of C2H3C1and some bond dissociation energies of C2H3CI and CHC1Fz will be given. 1. INTRODUCTION Photoionization of isolated molecules produces photoelectrons, phtoions or ionic and neutral fragments and even fluorescence photons. By detecting and analyzing these products one can obtain a number of important data for the molecules, such as state energies, absorption cross sections, ionization efficiencies, ionization potentials, dissociation energies, energy transfers, reaction channels and so on. Photoionization studies need an exciting light source which can provide photons in the range of 10-100eV for outer shell excitation and 100-1000eV for inner shell excitation of molecules. Synchrotron radiation meets these requirements very well. A photon beamline and an experimental station have been built at NSRL for these purposes. Photoionization studies of molecules can be done at the station by a few methods including molecular beam photoionization mass spectrometry, threshold photoelectron spectroscopy and threshold photoelectron-photoion coincidence, and useful results have been achieved for the last two years. In this paper we describe the photoionization of C2H3CI and CHCIFz using molecular beam photoionization mass spectrometry. 2. E X P E R I M E N T A L The experimental setup was described previously elsewhere, tll In brief, synchrotron radiation from the fifth bending magnet of the Hefei
800 MeV storage ring is focused by a toroidal mirror onto the entrance slit of a Im Seya-Namioka monochromator. The monochromator disperses the radiation in the range of 350-3000/~ with resolving power of greater than 500 with 220pm entrance and exit slits. The monochromatized radiation is focused by another toroidal mirror into the photoionization chamber. A lithium fluoride cutoff filter of 1 mm in thickness can be inserted into the beam path to eliminate second- and higher-order radiation when experiments are done at wavelengths longer than 1050A. The molecular beam is produced by expanding the sample gas, seeded in the carrier Ar or Ne gases through a 701am nozzle into the beam source chamber and later through a l mm skimmer into the photoionization chamber. It intersects the monochromatized synchrotron radiation beam at 70 mm from the nozzle. The photoionizaton chamber can be installed with two analyzers above and below the intersecting point. A experiment can be done with one or two of the prepared analyzers including a threshold photoelectron analyzer, a TOF and two quadrupole mass spectrometers and a photomultiplier depending on the requirements of the experiment. The photoionization and beam source chambers are pumped with a 1500 1/sec turbomolecular pump separately, the background pressure is 10 -4 Pa for the former and 102 Pa for the latter when the molecular beam is on. The setup is equipped a computerized control and data acquisition system. Experiments described here were performed only using one of the quadrupole mass spectrometers.
' This work was supported by the National Natural Science Foundation of China. 0368-2048/96/$15.00© 1996 ElsevierScienceB.V.All rights reserved PII S0368-2048 (96) 02900-3
484
The sample were C2H3CI of 99.9% purity and CHC1F2 of 99%. The pressure ratio of the samples to Ne was 1:5 to I:10 and the stagnation pressure was 1 to 3 bars. The monochromator was usually scanned with a wavelength increment of 0.20.5A and the data acquisition time for each point was I 0 to 20 seconds depending on ion abundance. 3. RESULTS
AND
",. ' : ":....;.
H+
Ca)
"..,"2' "
"
"~.",~. 665.7A
DISCUSSION
.......
3.1 Photoionization of C2H3C1 Fig.1 shows the PIE curve for C2H3C1+ from C2H3C1. The steep onsets at 9.98_+0.01eV and 11.66 + 0.05 eV are the ionization potentials corresponding to the ground and first excited electronic states of C2H3C1+. The energy difference 1.68 eV between these two states is in good agreement with our theoretical calculation value of 1.68 eV. The autoionization structure of the PIE curve in the range of 1060-1170A can be seen clearly. They were assigned as three Rydberg series ns, np and nd converging to the first excited state 2A' of C2H3C1+ and their quantum defects ~5(ns), 6(np) and 6(nd) were deduced to be 1.87, 1.51 and 0.22 respectively. 87
6
600
620
640
660
680
700
Wavelength/A
i,.',
C2H2C1÷
:.,,:,.;. (b)
'f.d" ,, 832.0A
7-'?,, . ::?'.? ....
""":"?:".',:-.~,.,.
7 3 0 .... 7~70~ ...... 8i(J . . . . . . . 850~ Wavelength/A
5
(c)
~ - " " ~ n s
C2H3 +
%,
% ~
600
690
780
870
988.9A
960 1050
Wavelength/A Fig. 2
illlllll~ 1056 ' ' '110() ' ' '1150
1200
PIE curves for H +, CzHzCI+ and CzH3+ from CzH3CI.
1250
Wavelength/A Fig. 1 PIE curve for C2H3C1÷ from C2H3C1 The appearance potentials of the fragment cations H +, CzH2C1+ and C2H3+ were measured to be 18.63+ 0.05 eV, 14.90+0.05 eV and 12.54+0.05 eV from their PIE curves ( shown in Fig. 2 (a, b, c) )
respectively. A few bond dissociation energies of C2H3C1 and C2H3C1÷ were evaluated from these data. The C-H bond dissociation energy Do(R-H) of a hydrocarbon can be determined by the following relation according to Field et al. [2] and Shiromaru et al.,[3] R-H + hv---~R + H++ e (1) Do(R-H)=AP(W)- IP(H) (2)
485 where the AP(H ÷) is the appearance potential of the fragment cation H ÷ from C2H3C1 and the ionization potential IP(H) of the H atom is well known as 13.598 eV, so we have Do(C2H~C1-H)=AP(H+)-IP(H)=5.03+0.05eV (3) The interference to the measurement of AP(H +) from the reaction channel R-H+hv-->R- + H + was neglected because we did not detect the C2HzCI in our experiment. The dissociation energies Do(CzHzCI+-H), Do(C2HzCI-H+) and Do(CzH3+-C1) of the molecular cation can be determined in the same way as above, Do(CzHzCI+-H)=AP(CzH2CI+)-IP(C2H3C1) =4.92+0.05 eV (4) Do(CzHzCI-H+)= AP(H+)-IP(CzH3CI) =8.65_+0.05 eV (5) Do(C2H3+_CI)=AP(C2H3+) - IP(C2H3C1) =2.56+0.05 eV (6) To determine the dissociation energy Do(CzH3-CI) the ionization potential of the radical C2H3 , IP(C2H3,), is needed. We failed to measure it. Li et al. t41 suggested that the most reasonable value of IP(CzH3,) should be 6.20 eV, so we have Do(C2H3-CI)-- AP(C2H3+) - IP(C2H3) --6.34+0.05 eV (7) The interference to the measurement of AP(C2H3+) from the channel C2H3CI + hv--> C2H3++ CI was omitted because we did not detect the ion CI. The above results are consistent with our theoretical calculation ones very well. I4]
4. S U M M A R Y
Photoionization studies of C2H3C1 and CHCIF 2 molecules were performed at NSRL by using the molecular beam photoionization mass spectrometry. A few Rydberg series of C2H3C1 were observed and some bond dissociation energies of C2H~CI, CHC1F 2 and their cations evaluated. .,
--
_
_ _
CHCIF2 +
(a)
. ...-..........~.. ~ •.
• • ..... "....."
~.
L"
12.16eV
940
960
980 1000 1020 Wacelength/~
1040
Cl+ i
. . . . .
•"v--.
(b) ""
i "..
I
'
18.51eV
I
'""-!..-..-..~--..-.-.-..-.....i 3.2 Photoionization of CHCIF2 PIEs for the formation of CHC1F2÷, C1+ from CHC[F 2 are displayed in Fig.3 (a, b). From these curves, the ionization potential for CHC1F2 and the appearance potential for C1+ can be observed: IP(CHC1F2)=12.16+ 0.02 eV and AP(CI+)=18.51+ 0.05eV. With these values, the dissociation energy, D0(CI+-CHF2)can be easily calculated, Do(CI+-CHF2)=AP(CI+)- IP(CHC1F2) =6.35+0.05eV (8) APs of CC1F2+, CHCIF +, CC1F÷, CHFz+, CF2+, CCF, CHCI +, HCI+, CF + and CHF + from CHC1F2 were also measured, their values are 14.19, 14.28, 15.00, 12.39, 18.79, 18.08, 19.87, 14.94, 15.90 and 18.50eV respectively. Some dissociation channels will be discussed by comparing with theoretical calculations in the future.
550
580
610 640 670 700 Wacelength/A Fig.3 PIEs of(a) CHCIF2 + and (b) CF from CHCIF 2. REFERENCES
I.Y.W. Zhang, L. S. Sheng, D. Q. Wang and T. Li, Chinese J. Chem. Phys. 5(5)(1992)321. 2 . F . H . Field and J. L. Franklin, ElEctron Impact
Phenomena and the Properties of Gaseous Ions, Academia Press, 1970, pp. 141-145. 3.H. Shiromaru, Y. Achiba, K. Kimura and Y. T. Lee, J. Phys. Chem. 91(1987)17. 4.L.S. Sheng, F. Qi, L. Tao, Y. W. Zhang, S. Q. Yu, C. K. Wong and W. K. Li, Int. J. Mass Spectrum. Ion Proc. (accepted).