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Chemiionization and secondary ion-molecule reactions in CH3OH and CH3OD
_formed-in piimary id s&&n~:procd~ on in&d of i&JS helium m&&ble~ atoms yith CH&H rind-C&OD, haye been studied. by me+ag_.@e $qj&&i+ pf .+F ionic products as a fun$tion-of CH&H__a+ CI$OD pressure using-a single-so-. mass $zctrom&~. Nti-titi b<&e io&$ obsetikd_~cq&aitiecihe&&_ The’ primary i&s&ctr& f&r the ~chemiibnization of ‘CHiOH-
:
INTRODUCTION
_.-
Chemiionization may be defined as ionization resulting from binary cdllisions of neutral atoms or molecules at thermal collision energies;-The energy required for ionization is supplied from internal excitation energy of one or both of- the reactants or--from-the.energy releasedin bond:formalioti or from a combination of thetwo& contimmtionof measur&ne&sreported earlier [I] on the khemiionizationof_H,O.and-D20;A@ieshave rick-bee6 made in this laboratory on the chemijonizationof -CH,OH- and CH,OJ?on impact of me&+&ablehelium atoms in the 2je’S states..These-two statescarry‘ ex&ation energies of 19.82 and 20.61 eV, respectively; so that, not-only ionization, but also extensiveGagmentation of-reactantmolecules.ispossible,.-. Prkaky-r-ioF- thusLformed.maythen undergo5on~ol&ule r&&ions le&i.ug to seconw. ions.-~VeryJittleWork hag beenzddn&6n%hechekiibnization~of polj&on$~i molecules;iPrevietis!of @evious.;work on: atomi&-anddiitomic sy,&ms
may: be found .,
:_ -_ IL_z >:j:~;~:<.c_;~_y:_~:~;:T;; -2.;5.:: .1.:I --;;I-.::_-1_.: y-2’ ~I.._ _-‘. /:J :~ c;_:z;~._.._~ -;,; : 1-.: ... _ ? .~i . .--- _x = --- _; I- ___.~ ___._= .- -..- - _~
&;RefL_lg
:-
EXE’ERIMENTAL METHOD
The apparatushas been describedpreviouslyby Kramer et al_ [Z] _ A beam of met&able helium atoms passes through a collision chamber.‘filledwith target gas at pressures of O-l-l-4 mtorr as determined using a precisely calibrated thermocouple gauge installed within the collision chamber. The beam is excited by 8OeV electrons [3] in a source chamber located about 10 cm from the collision chamber. Ions are extracted from the collision . chamber at right angles to the beam path by a draw-out field of 4.2 V. cm_’ which resultsin a terminal ion energy of 2.7 eV for all ions in th& study. The extracted ions entera magnetic-deflectionmassspectrometerand are detected by a 20-stage electron multiplier_Multiplier output pulses are amplified and counted for 40-60 s while the mass spectrometeris focused on the peak of a desiredion_ The helium used was obtained from -Air Reduction Company and was rated at better than 99.9% purity. The methanol used was Malhnckrodt reagent grade. The methanol-d was obtained from International Chemical and Nuclear Corporation and rated as 99 atom%_ Both materials were degassedthoroughly before use. The procedure in obtaining the experimental data was the same as used before [ 1] _ Cross-sectionsfor the secondaryion-molecule reactionsobserved were obtained using the equations
(1) or
where o is the cross-section, 2 the reaction path length, n, the number density of target molecules per unit pressure, P the target gas pressure, Ip and Is the primary and secondary ion signals, and 1: the primary ion signal extrapoksted to zero pressure_ RESULTS
Plots of percentage ion abundance vs. collision chamber pressure for the metastable helium+H~OH system are shown in Figs. 1 and.2. Similarplots were made for the ions observed with CH,OD_ It is .apparentthat all ions .: ~. observedare primaryions formed by chemiionization, e-g_ He* (23*1.5Z) + CH30H + He(l’S) + CH30’ + H + e
with the exception of (CHAOS). All the -data e&bit
a linear pre&uredependence indicating the absence of tert&y iou fokation, kl the prim&y ions with the exception of (Cg) appear6 react in secondary ion-molecule
- _
-_
.z..
W 0
.a,
_
-._.
..
W’
a IO
I
I
0.1
0.2
I 03
I 0.4
1 I I I 0.5 0.6 0.7 0.8 tmiilitorr) -. PRESSURE
I 0.9
-1’
I
I.0
1.1
-..
~
-~-
Fig. 1. Relative abundance of CHS, CHO+, and CHsO+ vs. collisiqn cham& . the &eoefion ofCH30H_ ~. ~: r _~.
.:.
._~.
pressure in .:
_.-
.-
CH,OH+-
.
.-
_%,.
5 0
.W
a
._
II
m
”
.
I 02
I a3
1
I
0.4 .-o-5
I’.!0.6.'-03 i
n
CH20+
..
/ ! 0.1
..
r\
I O-8
..
-_
: -_ I
--‘I-:
o+:_ co:-- 1.1.
. _ . -_
58 TABLE 1. Percentage ion abundance of primary ion spectra
l
e + CHxOH
_
-
Statistical theory
Ion
He* + CHJOH
CH:
CH; co*
12 12.1 0.5
I -5 8.5 5.5
7 -
CHO+ CH;! 0’
26.5 4.4
19.5 2.6
31 -
CHsO+ CH30H+
40.3 14.8
37-2 25.2
40 22
--
* The results given in the last two cohunns are taken from Ref_ 4. Experimental data in the second column do not add up to 100% because of uncertainty in. the extrapolated results. Estimated error in the abundance measurements is *0.5_
reactions, e.g.
CHXOH’ + CH30H + CH30s
-i-CH30
The CE ion formed from C&OD was equally unreactive. In the case of CH30D both secondary ions, CH,OD; and CH,ODH’, were observed, The data were extrapolated to zero pressure to obtain the primary ion spectra shown in Tables 1 and 2. Ambiguity exists for both masses 30 and 31 as shown in Table 2. With the low signal intensities of these experiments, the mass spectrometer could not be used with sufficient remlution to separate ions at the same mass numbers Because of the large abundance of ions at masses 29 and 32, it is very likely that the mass-30 ion is predominantly COD’ and the mass-31 ion predominately CH30’. Retarding potential measurements showed that aLI ions except CH‘; had thermal kinetic energy.
TABLE 2 Primary ion spectrum:
14 15 28 29 30 31 32 33
He’ + CH30D
Ion
Percent abundance
;s
1.5 11.9 0.4 20.7 5.3 9.4 36-4 12.7
co:
CHO+ CHz 0’; COD+ CHsO+; CHOD+ CHaOD+ CHsOD+
: ..
-_
-- z .-
..
59
:; :: I.‘ r _.:_-‘. _. : _-
DISCUSSION
-The results in -Table l- are compared -with -the.I,I;kctron-imp~~t-~-spe (&.I@~~ 3j of- l%iedman.tit al. -[4]tid .__._. _with ._ _ _their calculations _--.._~ -. (colur6.4)~ __ . . ._--._._ -.: -b-&;eddn ihe. &t&i&&l &-&y‘df I&S spectra [S]_ n& agreeme&.” r-a+; able. Evidently, the presence of the helium atom-do& li&l& td -p@urG- the. decomposition-of the excited parent ion in the che&ic&zation process.One‘ significa@ difference is the low. abundance of CO.*-in_$h& &hk&ionizatioe -m;Fe-:~ spectrum& compared to that for el&zon impact. ._ I..‘__-I_--_ I :_‘;I .: ~. Associativeionization(He* + CH,OH 3 H&.H30sy t me) &ndkrrange&ent-ionization (He* + CH,OH --, HeH* -+ CH@ + e) were. nOt;~observkl in t&se experiments, A definite trend with the size of the molecule’k obserkd, as shown in Table 3. The absence of associativeio&&ion is expected in pply atomic systems,.since it is difficult to conserve angular ~o&e&um in s&zh ___~. __ ._ collisions. Cross-sections for the secondary ion-molecule reactions observed are shown in Table 4; These are calculated on the- basis of two assumptions. Using the first (A), the cross&&ion is calcu.la&dfrom -the-j&&$~& thesecondary ion abundance with pressureusing eqn. (1). This ‘givksthe ‘cross~ sectionfor the reaction
reaction is the ody source.of CH,O& Using the second assumption (B), the cross-sectionis calculated.fiom the dkpletion~inthe ahtid*ce~ of the pzcixmxy ion using eqn. (2). .Thisassumptiongivesthe-correctresult only if there are no charge trar&e.Preactions forming the p&e ion, .The two cross-sectionsdo not agke so that the rea&ons of the other -primaryions .. .. lead to CH;Ow’ ok CH30H;.prodtiction or b_oth,e.g. . .- .: ~. CHO* + CH30H it k&OH i CHO, charge transfer ~. if this
+ cI-i,oH; .. .
+‘C$
pro+&ansfer
-
. .
. ~:.
_
-::
The &u&ion is similar for the CHJOD resGlt5.Both-secondary ions~are~ob--:
60 TABLE 4 Phenomenological cross-sections Assumption *
F!kaction CH30HC +- CH30H +CY&OH; + .__ C&OH + CHsOH * CH#H; + _.. CHO’ + CH30H + Products CH20+ + CH30H + Products CH30+ + CH30H + Products CHSOD+ + CH30D + CE130D; +- . .. CHSOD+ + CH30D + CHSODH+ + ___
CHsOD++CH#D
+
CHO++ CH30D + Products C&OD+ + CH30D + Products
._.
Cross-se&ion (A2) 538
A A
143 93 134 69 288. 303
B
87
-
94 64
; -
l*
-(124). (90) (67) (78)
* Assumption Ar the reaction shown is the only source of the product ion. Assumption B: the reactant ions are all primary - none are generatedby charge transfer reactions. ** Numbers in parentheses in the last column have been calculated from the rate constant measurementsof Ref. 7.
in this case and the cross-sectionfor their formation by whatever process(es)Is the same within experimental error_ In thelastcolumn of Table 3, the numbersin parenthesesare cross-sections. calculated from the results of Gupta et al. [ 7]_ These were obtained by electron impact using a single-sourcemass spectrometer tith a terminal ion energy of 3.4 eV. The agreementis quite satisfactorywith the exceptionof the cross-sectionfor depletion of the CH20’ ion. This ion is formed in low abundance and consequently the cross-sectionis known less accurately.It may therefore be concluded that, in the chemiionizationof methanol on impact of metastable helium atoms, the -primaryions are formed not only with essentiallythe same relative abundance but also with nearly the same degree of internal excitation, for otherwise the cross-sections for the secondaryi\jnmoIecule reactionswould depend upon the method of ionization. The same conckion is reachedin the case of the secondaryion-molecule reactionsobserved to followthe chemiionizationof H,O [l] ifthe electronim-_ pact measurementof Gupta et al. [ 71 isused_for comparisonratherthan thaitof Ryan [S]. Thus, the chemiionizationof thesepolyatomic m&c&s appears to take place under conditions such that there is littIe perturbation of the molecule or molecular ion by the helium atom in the collision. This is consistent with analysesof the energy of the electronsproduced in the chemiionization of many diatom5 moleculeswhich show that the molecw_ion is produced with a distribution of vibrational kvels vezz nearly the. *e:as thatobservedinphotoionization[9]. _ .-. ~. ._~ -L 1-_-: served
-
:
R.A. Sanders ~%I&E-E_ Muschlitz, Jr., I&J. Mass Spectroti Ion Phys, 23 (1977) 99. H-L_ Kramer; J.A. Herce and E-E, Muschlitz, Jr., J. Chem. Phys.. .56. (19i2) 4166.. This gives rise to a ratio of 2l~/2~S He* of 2/l. J.L.G. Dugan, XL,L; Richards and E.E. Muschlitz, Jr., J. Chem. Phys, 46 (1957) 346. .~. L. Friedman, F.A Long and M. Wolfsberg, J. Chem. Phys.,.27 (1957) 613 H-M. Rosetitock, M.B. Wallensteik A-L. Wahrh&%g and Henry. Eyring, Pro& Nat. Acad. Sci. U.S.A., 38 (1952) 667. L.T. Specht,.K.D. Foster and E-E. Muschlitz, Jr., E Ch&_ Phys., 63 (1975) 1582 SK. Gupta, E.G. Jones, AG. Harrison and J.J. Myher, Can. 3. Chem., 45 (1967) K-R. Ryan, J. Chem. Phys-, 52 (1970) 6009. Hi_ Hotop, Radiat. Res., 59 (1974) 379:
_
3167.
:
INTRODUCTION
_.-
Chemiionization may be defined as ionization resulting from binary cdllisions of neutral atoms or molecules at thermal collision energies;-The energy required for ionization is supplied from internal excitation energy of one or both of- the reactants or--from-the.energy releasedin bond:formalioti or from a combination of thetwo& contimmtionof measur&ne&sreported earlier [I] on the khemiionizationof_H,O.and-D20;A@ieshave rick-bee6 made in this laboratory on the chemijonizationof -CH,OH- and CH,OJ?on impact of me&+&ablehelium atoms in the 2je’S states..These-two statescarry‘ ex&ation energies of 19.82 and 20.61 eV, respectively; so that, not-only ionization, but also extensiveGagmentation of-reactantmolecules.ispossible,.-. Prkaky-r-ioF- thusLformed.maythen undergo5on~ol&ule r&&ions le&i.ug to seconw. ions.-~VeryJittleWork hag beenzddn&6n%hechekiibnization~of polj&on$~i molecules;iPrevietis!of @evious.;work on: atomi&-anddiitomic sy,&ms
may: be found .,
:_ -_ IL_z >:j:~;~:<.c_;~_y:_~:~;:T;; -2.;5.:: .1.:I --;;I-.::_-1_.: y-2’ ~I.._ _-‘. /:J :~ c;_:z;~._.._~ -;,; : 1-.: ... _ ? .~i . .--- _x = --- _; I- ___.~ ___._= .- -..- - _~
&;RefL_lg
:-
EXE’ERIMENTAL METHOD
The apparatushas been describedpreviouslyby Kramer et al_ [Z] _ A beam of met&able helium atoms passes through a collision chamber.‘filledwith target gas at pressures of O-l-l-4 mtorr as determined using a precisely calibrated thermocouple gauge installed within the collision chamber. The beam is excited by 8OeV electrons [3] in a source chamber located about 10 cm from the collision chamber. Ions are extracted from the collision . chamber at right angles to the beam path by a draw-out field of 4.2 V. cm_’ which resultsin a terminal ion energy of 2.7 eV for all ions in th& study. The extracted ions entera magnetic-deflectionmassspectrometerand are detected by a 20-stage electron multiplier_Multiplier output pulses are amplified and counted for 40-60 s while the mass spectrometeris focused on the peak of a desiredion_ The helium used was obtained from -Air Reduction Company and was rated at better than 99.9% purity. The methanol used was Malhnckrodt reagent grade. The methanol-d was obtained from International Chemical and Nuclear Corporation and rated as 99 atom%_ Both materials were degassedthoroughly before use. The procedure in obtaining the experimental data was the same as used before [ 1] _ Cross-sectionsfor the secondaryion-molecule reactionsobserved were obtained using the equations
(1) or
where o is the cross-section, 2 the reaction path length, n, the number density of target molecules per unit pressure, P the target gas pressure, Ip and Is the primary and secondary ion signals, and 1: the primary ion signal extrapoksted to zero pressure_ RESULTS
Plots of percentage ion abundance vs. collision chamber pressure for the metastable helium+H~OH system are shown in Figs. 1 and.2. Similarplots were made for the ions observed with CH,OD_ It is .apparentthat all ions .: ~. observedare primaryions formed by chemiionization, e-g_ He* (23*1.5Z) + CH30H + He(l’S) + CH30’ + H + e
with the exception of (CHAOS). All the -data e&bit
a linear pre&uredependence indicating the absence of tert&y iou fokation, kl the prim&y ions with the exception of (Cg) appear6 react in secondary ion-molecule
- _
-_
.z..
W 0
.a,
_
-._.
..
W’
a IO
I
I
0.1
0.2
I 03
I 0.4
1 I I I 0.5 0.6 0.7 0.8 tmiilitorr) -. PRESSURE
I 0.9
-1’
I
I.0
1.1
-..
~
-~-
Fig. 1. Relative abundance of CHS, CHO+, and CHsO+ vs. collisiqn cham& . the &eoefion ofCH30H_ ~. ~: r _~.
.:.
._~.
pressure in .:
_.-
.-
CH,OH+-
.
.-
_%,.
5 0
.W
a
._
II
m
”
.
I 02
I a3
1
I
0.4 .-o-5
I’.!0.6.'-03 i
n
CH20+
..
/ ! 0.1
..
r\
I O-8
..
-_
: -_ I
--‘I-:
o+:_ co:-- 1.1.
. _ . -_
58 TABLE 1. Percentage ion abundance of primary ion spectra
l
e + CHxOH
_
-
Statistical theory
Ion
He* + CHJOH
CH:
CH; co*
12 12.1 0.5
I -5 8.5 5.5
7 -
CHO+ CH;! 0’
26.5 4.4
19.5 2.6
31 -
CHsO+ CH30H+
40.3 14.8
37-2 25.2
40 22
--
* The results given in the last two cohunns are taken from Ref_ 4. Experimental data in the second column do not add up to 100% because of uncertainty in. the extrapolated results. Estimated error in the abundance measurements is *0.5_
reactions, e.g.
CHXOH’ + CH30H + CH30s
-i-CH30
The CE ion formed from C&OD was equally unreactive. In the case of CH30D both secondary ions, CH,OD; and CH,ODH’, were observed, The data were extrapolated to zero pressure to obtain the primary ion spectra shown in Tables 1 and 2. Ambiguity exists for both masses 30 and 31 as shown in Table 2. With the low signal intensities of these experiments, the mass spectrometer could not be used with sufficient remlution to separate ions at the same mass numbers Because of the large abundance of ions at masses 29 and 32, it is very likely that the mass-30 ion is predominantly COD’ and the mass-31 ion predominately CH30’. Retarding potential measurements showed that aLI ions except CH‘; had thermal kinetic energy.
TABLE 2 Primary ion spectrum:
14 15 28 29 30 31 32 33
He’ + CH30D
Ion
Percent abundance
;s
1.5 11.9 0.4 20.7 5.3 9.4 36-4 12.7
co:
CHO+ CHz 0’; COD+ CHsO+; CHOD+ CHaOD+ CHsOD+
: ..
-_
-- z .-
..
59
:; :: I.‘ r _.:_-‘. _. : _-
DISCUSSION
-The results in -Table l- are compared -with -the.I,I;kctron-imp~~t-~-spe (&.I@~~ 3j of- l%iedman.tit al. -[4]tid .__._. _with ._ _ _their calculations _--.._~ -. (colur6.4)~ __ . . ._--._._ -.: -b-&;eddn ihe. &t&i&&l &-&y‘df I&S spectra [S]_ n& agreeme&.” r-a+; able. Evidently, the presence of the helium atom-do& li&l& td -p@urG- the. decomposition-of the excited parent ion in the che&ic&zation process.One‘ significa@ difference is the low. abundance of CO.*-in_$h& &hk&ionizatioe -m;Fe-:~ spectrum& compared to that for el&zon impact. ._ I..‘__-I_--_ I :_‘;I .: ~. Associativeionization(He* + CH,OH 3 H&.H30sy t me) &ndkrrange&ent-ionization (He* + CH,OH --, HeH* -+ CH@ + e) were. nOt;~observkl in t&se experiments, A definite trend with the size of the molecule’k obserkd, as shown in Table 3. The absence of associativeio&&ion is expected in pply atomic systems,.since it is difficult to conserve angular ~o&e&um in s&zh ___~. __ ._ collisions. Cross-sections for the secondary ion-molecule reactions observed are shown in Table 4; These are calculated on the- basis of two assumptions. Using the first (A), the cross&&ion is calcu.la&dfrom -the-j&&$~& thesecondary ion abundance with pressureusing eqn. (1). This ‘givksthe ‘cross~ sectionfor the reaction
reaction is the ody source.of CH,O& Using the second assumption (B), the cross-sectionis calculated.fiom the dkpletion~inthe ahtid*ce~ of the pzcixmxy ion using eqn. (2). .Thisassumptiongivesthe-correctresult only if there are no charge trar&e.Preactions forming the p&e ion, .The two cross-sectionsdo not agke so that the rea&ons of the other -primaryions .. .. lead to CH;Ow’ ok CH30H;.prodtiction or b_oth,e.g. . .- .: ~. CHO* + CH30H it k&OH i CHO, charge transfer ~. if this
+ cI-i,oH; .. .
+‘C$
pro+&ansfer
-
. .
. ~:.
_
-::
The &u&ion is similar for the CHJOD resGlt5.Both-secondary ions~are~ob--:
60 TABLE 4 Phenomenological cross-sections Assumption *
F!kaction CH30HC +- CH30H +CY&OH; + .__ C&OH + CHsOH * CH#H; + _.. CHO’ + CH30H + Products CH20+ + CH30H + Products CH30+ + CH30H + Products CHSOD+ + CH30D + CE130D; +- . .. CHSOD+ + CH30D + CHSODH+ + ___
CHsOD++CH#D
+
CHO++ CH30D + Products C&OD+ + CH30D + Products
._.
Cross-se&ion (A2) 538
A A
143 93 134 69 288. 303
B
87
-
94 64
; -
l*
-(124). (90) (67) (78)
* Assumption Ar the reaction shown is the only source of the product ion. Assumption B: the reactant ions are all primary - none are generatedby charge transfer reactions. ** Numbers in parentheses in the last column have been calculated from the rate constant measurementsof Ref. 7.
in this case and the cross-sectionfor their formation by whatever process(es)Is the same within experimental error_ In thelastcolumn of Table 3, the numbersin parenthesesare cross-sections. calculated from the results of Gupta et al. [ 7]_ These were obtained by electron impact using a single-sourcemass spectrometer tith a terminal ion energy of 3.4 eV. The agreementis quite satisfactorywith the exceptionof the cross-sectionfor depletion of the CH20’ ion. This ion is formed in low abundance and consequently the cross-sectionis known less accurately.It may therefore be concluded that, in the chemiionizationof methanol on impact of metastable helium atoms, the -primaryions are formed not only with essentiallythe same relative abundance but also with nearly the same degree of internal excitation, for otherwise the cross-sections for the secondaryi\jnmoIecule reactionswould depend upon the method of ionization. The same conckion is reachedin the case of the secondaryion-molecule reactionsobserved to followthe chemiionizationof H,O [l] ifthe electronim-_ pact measurementof Gupta et al. [ 71 isused_for comparisonratherthan thaitof Ryan [S]. Thus, the chemiionizationof thesepolyatomic m&c&s appears to take place under conditions such that there is littIe perturbation of the molecule or molecular ion by the helium atom in the collision. This is consistent with analysesof the energy of the electronsproduced in the chemiionization of many diatom5 moleculeswhich show that the molecw_ion is produced with a distribution of vibrational kvels vezz nearly the. *e:as thatobservedinphotoionization[9]. _ .-. ~. ._~ -L 1-_-: served
-
:
R.A. Sanders ~%I&E-E_ Muschlitz, Jr., I&J. Mass Spectroti Ion Phys, 23 (1977) 99. H-L_ Kramer; J.A. Herce and E-E, Muschlitz, Jr., J. Chem. Phys.. .56. (19i2) 4166.. This gives rise to a ratio of 2l~/2~S He* of 2/l. J.L.G. Dugan, XL,L; Richards and E.E. Muschlitz, Jr., J. Chem. Phys, 46 (1957) 346. .~. L. Friedman, F.A Long and M. Wolfsberg, J. Chem. Phys.,.27 (1957) 613 H-M. Rosetitock, M.B. Wallensteik A-L. Wahrh&%g and Henry. Eyring, Pro& Nat. Acad. Sci. U.S.A., 38 (1952) 667. L.T. Specht,.K.D. Foster and E-E. Muschlitz, Jr., E Ch&_ Phys., 63 (1975) 1582 SK. Gupta, E.G. Jones, AG. Harrison and J.J. Myher, Can. 3. Chem., 45 (1967) K-R. Ryan, J. Chem. Phys-, 52 (1970) 6009. Hi_ Hotop, Radiat. Res., 59 (1974) 379:
_
3167.