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A new method for the measurement of vibrational-state-selected ion-molecule reactions at thermal energies
Vohtmc 96. number I
25 March 1983
CHEMICAL PHYSKS LETTERS
A NEW METHOD FOR THE MEASUREMENT OF VIB~TIONAL-STATE~ELE~ED D. VAN PIJKEREN.
ION-MOLECULE REACTIONS AT THERMAL ENERGIES
J. V.4N ECK and A. NIEHAUS
Ifssisclr I.aboratoriunl dcr Rtjknutivcrsiteit
Utrccht. I-Wncetonplein 5. 3584 CC Utrecht. The Nerkerlands
A ~‘l~otocicrtrc~n-se~onddr) -ion-coittcidcncc method is described that aiIows US to determine the relative vibrationalencrs)-dcpcndcrtf crtw sccttons for rcwttons of moiecul~r ions with neutr4 atoms or molccuIes at thermal energies. Rewlt~ for IL’JLIII~ of if;(u) m vtbr.rtion.d stdtcs LJ= O-8 with H2(H$). Ne(NeH+) and Hc(HeH+) arc reported.
1. Introduction
H;(U) + He -* HeH* + H
Two methods to study the influence of vibrational e~ci~.~ti~m on the reaction probability in ion-molecule
for vibrational
collisions are prcscntty appIied: The so-called TESICU ttiethod (tl~rcst~olditlcctron --secondary-ion-coinci-
dcucc method) 11] . and ;I method which combines f’lrirttliitnilJ!iuIl 2nd a m&o-frequency guided ion technique 111 .?‘hc ldttcr method relics on preferential popu1a1iou of a ccrtxirt vibrational state of the photoitrtii02d 1110lccuIc at 3 certain wsvelcngtli used for photcGottiL;itiott [2]_ .md lm been limited to reactions with II: ions. Both methods are limited to collision cncrgics above a few tenths of an cV. We have d~velaped 3 new method designed to measure at thermal cttcrgy where the mfluence of vibrational excitation is rqcctcd to bc strongest+ in this method the photoclcctron entitled in the ioniration event in which a primxy ion is formed is measured in coincidence with the srcortd+try ion created in a collision with some atom or mofeculc present in the ionization-reaction WIUSI~. In antiogy with the similar TESICO method, our method may be called the PESICO method (photoelsctron secondary-ion-coincidence method). fn this Ictter we give a short outline of the method, describe the essential pxts of the apparatus, and report first results obtained for the systems II;(u)tiIz-+fi;+ff. I-l:(u) + Ne + NeP 20
(Ia) + Ii .
(lb)
states from u = 0 to u = 8.
2. Apparatus and method A schematic drawing of the main parts of our apparatus is shown in fig. 1. The light source is a differ-
Fig. L Schematic of the apparatus. A: light source; 8; efectroty spectrometer; Cr accelenrion diaphragms and drift region; D: channel plate; Er photon beam detector_
0 009-2614/83/0000-0000/S
03.00 0 1983 North-Holland
CHEMICAL PHYSICS LETTERS
Volume 96, number 1
entially pumped dc discharge [4]. It can be run with the rare gases, and delivers predominantly the corresponding lowest resonance UV line. The light beam is collimated to a diameter of 1 mm in the ionization volume, which is formed by the overlap with the volume “seen” by the cylindrical mirror analyzer for the photoelectrons. This analyzer was designed using numerical material published for the case of finite source volumes [S]. A resolution of xl% at an angle of acceptance of =2% of 4s, for a source volume of -1 mm3 is achieved. Both the gas to be photoionized and the reactant gas are introduced into the ionizationreaction volume via a conical channel. At the tip of
the cone the density is enhanced by a factor of ~40 compared to the background density. Ions formed in the ionization volume may react in the reaction volume which encloses the ionization volume. The reaction volume is defied as the cylindrically shaped volume above an ion extraction hole of 3 mm diameter_ Ions present in this volume may be extracted by a triggered pulse of =90 V, are then post-accelerated, and, after passing a field-free drift space for space focusing [6], are finally detected by a micro-channel plate. This apparatus serves as a “time-of-flight” mass spectrometer_ The resolution achieved isM/AM = 40. To described the method that allows us to measure relative reaction probabilities for different vibrational states of the photoions, we consider, as an example, the system Hz/He: Both H, and He, are present in the field-free ionization-reaction volume. Ha is ionized
by the UV photons of fued energy hu, and lS&) is formed in various vibrational states with probabilities determined by the Franck-Condon factors, h v + H2 + Hz(u) + e-(eJ
_
(2)
The photoelectron corresponding to formation of Hz in a certain vibrational state (u) has a well-defined energy eu determined by energy conservation_ Let us assume an electron of energy eu is detected. At the tune of detection the corresponding ion Hz(u) is still very close to the position where it was formed. On the average it takes a few microseconds until it leaves the reaction volume. Within this time span there is a certain probability of collision whit a He atom, and to react ’ to give HeH+: Hz(v) -CHe 4 HeH+ + H _ Let us assume that reaction
25 March 1983
curred. An ion extraction pulse - applied after the reaction but before the secondary ion leaves the reaction volume - then leads to detection of the secondary ion in the “timeof-flight” mass spectrometer_ The sequence of events leading to the detection of a secondary HeH+ formed in a collision with a Hz in vibrational state u is clearly proportional to the probability of formation of the H$ in the vibrational state u in question, F(u), and proportional to the probability for reaction (3), h)(u), provided the density of parent and reactant gas is low enough to guarantee singlecollision conditions_ By scanning the electron spectrometer at otherwise fmed conditions, a “correlated” signal S(u) at the secondary ion mass is obtained which may be expressed as S(u) = GF(u) W(u)
(4)
with G a constant_ However, at any instant, there is also a certain probability for an “uncorrelated” ion to be present in the reaction volume_ For the parent ions this probability is given by the average residence time in the reaction volume times the rate of ionization processes_ For the secondary ions the probability is lower by a factor which might be called the average reaction probability_ An extraction pulse triggered by an electron, therefore, leads to detection of an uncorrelated signal at the secondary ion mass, in addition to the correlated signal (4)_ The uncorrelated signal is a pure reflection of the “singles” electron spectrum and may be expressed as S’(u) = CF(u)
(5)
with C again some constant_ The uncorrelated signal we measure directly by applying, after each extraction pulse triggered by an electron, a second pulse delayed long enough to guarantee “statistical conditions” in the reaction volume. The signal belonging to the first pulse, Stat = S(u) + S’(u), and the signal belonging to the “statistical” pulse, S’(u), are stored in different blocks of a multichannel analyzer whose channels in the blocks are advanced synchronously with a stepping voltage which scans the electron spectrometer. From the resulting two spectra we obtain the relative reaction probabilities: [St0 t(u) - S’(u)] /S’(u) = const . W(u) .
(6)
(3) (3) of our ion has oc21
25 March 1983
CHEhlICAL PHYSICS LETTERS
Volume 96. number 1
3_ Rest&s Fig. 2 shows, as an example, the spectra S&J) and S’(u) for the secondary ion HeHi formed in reaction (1 c). In fig. 3 we show the normalized reaction probabilities for the reactions (la)-flc) of Hs in the vibrational states O-8. Except for preliminary data reported for reaction (1 a) [ 7 1, these are the first such dara for rhe case of thermal collision energy. The reactions (1 b) and (1 c) are endother~c for u < 3 and u < 4, respectively [8 1. Since reaction (1 b) is mdothermic by only 20 meV for u = 2, this reaction becomes energetically possibIe through thermal trans-
I
B P 4
4
Fig. 3. Vibrational~ner~y~ependent relative cross sections for reactions of 11: u ith Hz (A). Ne (B) and He (C). The error bars indicate statistical errors.
Y
7
0
-
VIBRATfONAL
ENERGY
2. Typic.11 eumpfe of the measured spectra. A: non-coins~drnt elcrrron spectrum; B: correlated spectrum for HeH; C: uncorreLtcd spectrum for HeH. I$.
22
lational and rotational energy. The fact that for the other endothermic cases the measured reaction probability is not exact& zero must be attributed to reactions when the ions are accelerated by the extraction pulse. Another source of possible systematic error connected with our method is the occurrence of charge exchange between the vibrationally excited ion and the neutral parent gas. Except for the completely resonant process which has no influence, these processes lead to a lowering of the vibrational state and thus to a wrong labelling, In a reaction like (la) this would result in a density~ependent increase of the measured effective probabilities of the higher vibrational states. We have not observed such an effect.
Volume 96. number 1
CHEMICAL
PHYSICS
LElTERS
In conclusion we may state that we have developed a rather versatile method, free of large systematic errors, for the determination of thermal energy, vibrational-state-selected, ion-molecule reaction probabilities. Results for other systems, and a more detailed description of the method and of the apparatus used will be published in the near future.
References
Acknowledgement
[3]
This work was performed as part of the research programme of the “Stichting voor Fundamenteel Onderzoek der hlaterie” (F_O_M.) with financial support from the “Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek” (Z.W.O.).
[l]
[2]
[4] [5] [6] [7] [S]
25 March 1983
I. Koyano and K-Tanaka, J. Chem. Phys. 72 (1980) 4858; T. Kato, K_ Tanaka and 1. Koyano, J. Chem. Phys. 77 (1982) 337. S.L. Anderson, F-A. Houle, D. Gerlich and Y-T_ Lee, J. Chem. Phys. 75 (1981) 2153; S.L. Anderson, T. Turner, B.H. Mahan and Y-T. Lee. J. Chem. Phys. 77 (1982) 1842. P.M. Dehmer and WA. Chupka, J. Chem. Phys. 65 (1976) 2243. H. Haberland. University of Freibu%, private communication. J-E. Qraper and C. Lee, Rev. Sci. Intr. 48 (1977) 852. W-C. Wiley and T-H. McLaren. Rev. Sci. Intr. 26 (1955) 1150. AJ. Yencha. A. Miinzer and A. Niehaus, ICPEAC 1979, pp. 902 ff. P. Rosmus and E-A_ Reinsch, Z. Naturforsch. 35A (1980) 1066.
23
25 March 1983
CHEMICAL PHYSKS LETTERS
A NEW METHOD FOR THE MEASUREMENT OF VIB~TIONAL-STATE~ELE~ED D. VAN PIJKEREN.
ION-MOLECULE REACTIONS AT THERMAL ENERGIES
J. V.4N ECK and A. NIEHAUS
Ifssisclr I.aboratoriunl dcr Rtjknutivcrsiteit
Utrccht. I-Wncetonplein 5. 3584 CC Utrecht. The Nerkerlands
A ~‘l~otocicrtrc~n-se~onddr) -ion-coittcidcncc method is described that aiIows US to determine the relative vibrationalencrs)-dcpcndcrtf crtw sccttons for rcwttons of moiecul~r ions with neutr4 atoms or molccuIes at thermal energies. Rewlt~ for IL’JLIII~ of if;(u) m vtbr.rtion.d stdtcs LJ= O-8 with H2(H$). Ne(NeH+) and Hc(HeH+) arc reported.
1. Introduction
H;(U) + He -* HeH* + H
Two methods to study the influence of vibrational e~ci~.~ti~m on the reaction probability in ion-molecule
for vibrational
collisions are prcscntty appIied: The so-called TESICU ttiethod (tl~rcst~olditlcctron --secondary-ion-coinci-
dcucc method) 11] . and ;I method which combines f’lrirttliitnilJ!iuIl 2nd a m&o-frequency guided ion technique 111 .?‘hc ldttcr method relics on preferential popu1a1iou of a ccrtxirt vibrational state of the photoitrtii02d 1110lccuIc at 3 certain wsvelcngtli used for photcGottiL;itiott [2]_ .md lm been limited to reactions with II: ions. Both methods are limited to collision cncrgics above a few tenths of an cV. We have d~velaped 3 new method designed to measure at thermal cttcrgy where the mfluence of vibrational excitation is rqcctcd to bc strongest+ in this method the photoclcctron entitled in the ioniration event in which a primxy ion is formed is measured in coincidence with the srcortd+try ion created in a collision with some atom or mofeculc present in the ionization-reaction WIUSI~. In antiogy with the similar TESICO method, our method may be called the PESICO method (photoelsctron secondary-ion-coincidence method). fn this Ictter we give a short outline of the method, describe the essential pxts of the apparatus, and report first results obtained for the systems II;(u)tiIz-+fi;+ff. I-l:(u) + Ne + NeP 20
(Ia) + Ii .
(lb)
states from u = 0 to u = 8.
2. Apparatus and method A schematic drawing of the main parts of our apparatus is shown in fig. 1. The light source is a differ-
Fig. L Schematic of the apparatus. A: light source; 8; efectroty spectrometer; Cr accelenrion diaphragms and drift region; D: channel plate; Er photon beam detector_
0 009-2614/83/0000-0000/S
03.00 0 1983 North-Holland
CHEMICAL PHYSICS LETTERS
Volume 96, number 1
entially pumped dc discharge [4]. It can be run with the rare gases, and delivers predominantly the corresponding lowest resonance UV line. The light beam is collimated to a diameter of 1 mm in the ionization volume, which is formed by the overlap with the volume “seen” by the cylindrical mirror analyzer for the photoelectrons. This analyzer was designed using numerical material published for the case of finite source volumes [S]. A resolution of xl% at an angle of acceptance of =2% of 4s, for a source volume of -1 mm3 is achieved. Both the gas to be photoionized and the reactant gas are introduced into the ionizationreaction volume via a conical channel. At the tip of
the cone the density is enhanced by a factor of ~40 compared to the background density. Ions formed in the ionization volume may react in the reaction volume which encloses the ionization volume. The reaction volume is defied as the cylindrically shaped volume above an ion extraction hole of 3 mm diameter_ Ions present in this volume may be extracted by a triggered pulse of =90 V, are then post-accelerated, and, after passing a field-free drift space for space focusing [6], are finally detected by a micro-channel plate. This apparatus serves as a “time-of-flight” mass spectrometer_ The resolution achieved isM/AM = 40. To described the method that allows us to measure relative reaction probabilities for different vibrational states of the photoions, we consider, as an example, the system Hz/He: Both H, and He, are present in the field-free ionization-reaction volume. Ha is ionized
by the UV photons of fued energy hu, and lS&) is formed in various vibrational states with probabilities determined by the Franck-Condon factors, h v + H2 + Hz(u) + e-(eJ
_
(2)
The photoelectron corresponding to formation of Hz in a certain vibrational state (u) has a well-defined energy eu determined by energy conservation_ Let us assume an electron of energy eu is detected. At the tune of detection the corresponding ion Hz(u) is still very close to the position where it was formed. On the average it takes a few microseconds until it leaves the reaction volume. Within this time span there is a certain probability of collision whit a He atom, and to react ’ to give HeH+: Hz(v) -CHe 4 HeH+ + H _ Let us assume that reaction
25 March 1983
curred. An ion extraction pulse - applied after the reaction but before the secondary ion leaves the reaction volume - then leads to detection of the secondary ion in the “timeof-flight” mass spectrometer_ The sequence of events leading to the detection of a secondary HeH+ formed in a collision with a Hz in vibrational state u is clearly proportional to the probability of formation of the H$ in the vibrational state u in question, F(u), and proportional to the probability for reaction (3), h)(u), provided the density of parent and reactant gas is low enough to guarantee singlecollision conditions_ By scanning the electron spectrometer at otherwise fmed conditions, a “correlated” signal S(u) at the secondary ion mass is obtained which may be expressed as S(u) = GF(u) W(u)
(4)
with G a constant_ However, at any instant, there is also a certain probability for an “uncorrelated” ion to be present in the reaction volume_ For the parent ions this probability is given by the average residence time in the reaction volume times the rate of ionization processes_ For the secondary ions the probability is lower by a factor which might be called the average reaction probability_ An extraction pulse triggered by an electron, therefore, leads to detection of an uncorrelated signal at the secondary ion mass, in addition to the correlated signal (4)_ The uncorrelated signal is a pure reflection of the “singles” electron spectrum and may be expressed as S’(u) = CF(u)
(5)
with C again some constant_ The uncorrelated signal we measure directly by applying, after each extraction pulse triggered by an electron, a second pulse delayed long enough to guarantee “statistical conditions” in the reaction volume. The signal belonging to the first pulse, Stat = S(u) + S’(u), and the signal belonging to the “statistical” pulse, S’(u), are stored in different blocks of a multichannel analyzer whose channels in the blocks are advanced synchronously with a stepping voltage which scans the electron spectrometer. From the resulting two spectra we obtain the relative reaction probabilities: [St0 t(u) - S’(u)] /S’(u) = const . W(u) .
(6)
(3) (3) of our ion has oc21
25 March 1983
CHEhlICAL PHYSICS LETTERS
Volume 96. number 1
3_ Rest&s Fig. 2 shows, as an example, the spectra S&J) and S’(u) for the secondary ion HeHi formed in reaction (1 c). In fig. 3 we show the normalized reaction probabilities for the reactions (la)-flc) of Hs in the vibrational states O-8. Except for preliminary data reported for reaction (1 a) [ 7 1, these are the first such dara for rhe case of thermal collision energy. The reactions (1 b) and (1 c) are endother~c for u < 3 and u < 4, respectively [8 1. Since reaction (1 b) is mdothermic by only 20 meV for u = 2, this reaction becomes energetically possibIe through thermal trans-
I
B P 4
4
Fig. 3. Vibrational~ner~y~ependent relative cross sections for reactions of 11: u ith Hz (A). Ne (B) and He (C). The error bars indicate statistical errors.
Y
7
0
-
VIBRATfONAL
ENERGY
2. Typic.11 eumpfe of the measured spectra. A: non-coins~drnt elcrrron spectrum; B: correlated spectrum for HeH; C: uncorreLtcd spectrum for HeH. I$.
22
lational and rotational energy. The fact that for the other endothermic cases the measured reaction probability is not exact& zero must be attributed to reactions when the ions are accelerated by the extraction pulse. Another source of possible systematic error connected with our method is the occurrence of charge exchange between the vibrationally excited ion and the neutral parent gas. Except for the completely resonant process which has no influence, these processes lead to a lowering of the vibrational state and thus to a wrong labelling, In a reaction like (la) this would result in a density~ependent increase of the measured effective probabilities of the higher vibrational states. We have not observed such an effect.
Volume 96. number 1
CHEMICAL
PHYSICS
LElTERS
In conclusion we may state that we have developed a rather versatile method, free of large systematic errors, for the determination of thermal energy, vibrational-state-selected, ion-molecule reaction probabilities. Results for other systems, and a more detailed description of the method and of the apparatus used will be published in the near future.
References
Acknowledgement
[3]
This work was performed as part of the research programme of the “Stichting voor Fundamenteel Onderzoek der hlaterie” (F_O_M.) with financial support from the “Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek” (Z.W.O.).
[l]
[2]
[4] [5] [6] [7] [S]
25 March 1983
I. Koyano and K-Tanaka, J. Chem. Phys. 72 (1980) 4858; T. Kato, K_ Tanaka and 1. Koyano, J. Chem. Phys. 77 (1982) 337. S.L. Anderson, F-A. Houle, D. Gerlich and Y-T_ Lee, J. Chem. Phys. 75 (1981) 2153; S.L. Anderson, T. Turner, B.H. Mahan and Y-T. Lee. J. Chem. Phys. 77 (1982) 1842. P.M. Dehmer and WA. Chupka, J. Chem. Phys. 65 (1976) 2243. H. Haberland. University of Freibu%, private communication. J-E. Qraper and C. Lee, Rev. Sci. Intr. 48 (1977) 852. W-C. Wiley and T-H. McLaren. Rev. Sci. Intr. 26 (1955) 1150. AJ. Yencha. A. Miinzer and A. Niehaus, ICPEAC 1979, pp. 902 ff. P. Rosmus and E-A_ Reinsch, Z. Naturforsch. 35A (1980) 1066.
23