Thermal ionization of acetone peroxide on tungsten and on oxidized tungsten

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  • general properties of the electron-nuclear Coulomb potential. The positiveness of the first member in the right part of (4) is a consequence of the positive determinacy of the kinetic- energy operator T.

    LITERATURE CITED J

    I. I .B . Bersuker, "Origin of the dynamic instability of molecular systems," Teor. Eksp. Khim., 16, No. 3, 291-299 (1980).

    2. A .B . Anderson, N. C. Handy, and R. G. Parr, "Relationship between vibrational force constants and quadrupole coupling constants for molecules and solids," J. Chem. Phys., 50, No. 8, 3634-3635 (1969).

    3. ~. L. Clinton, "Forces in molecules. 2. A differential equation for the potential- energy function," J. Chem. Phys., 38, No. I0, 2339-2344 (1963).

    THERMAL IONIZATION OF ACETONE PEROXIDE ON TUNGSTEN AND ON

    OXIDIZED TUNGSTEN

    V. I. Paleev UDC 547.284.3-39+537.58.62-4+543.51

    Mass spectrometric analysis of organic peroxides and mixtures containing them is extreme- ly complicated, since, on account of the instability of these compounds, the vapors to be analyzed contain not only the peroxide but also endproducts formed in its thermal decomposi- tion and reactions on the walls and in the volume of the instrument. The detection of per- oxides in mass spectrometric analysis of mixtures is also hindered by the absence of molec- ular ions in the mass spectra of ionization by electrons. Therefore the search for other methods of ionization of these compounds is an urgent problem. One of these methods may be ionization of peroxides on the surface of heated solids. This work presents the results of an investigation of the thermal ionization of dimer (DPA) and trimer (TPA) peroxides of ace- tone on tungsten and on oxidized tungsten, with the formation of positive and negative ions:

    90,,/cH H3c,/o o\ lcH c c ; c - - c .

    / \o o ( I f ix ~-C \CH,~ H~C O0--C--O0 OH 3 H3C / \CH~

    DPA TPA

    As is shown in [I], when molecules of trimer acetone peroxide break down, radicals that are ionized by nonequilibrium ionization processes during thermal desorption are formed on the surface of heated tungsten or oxidized tungsten. Nonequilibrium ionization was manifested both in an increase in the yield of ions in comparison with that expected for the equilibrium process of surface ionization and in a deviation from the equilibrium energy distribution of the desorbed ions. The excitation of radicals in the case of their formation in the layer adsorbed on the surface apparently indicates a dual influence of ionization on them: It may decrease the effective ionization potential V' of the particle at the surface or correspond- ingly increase the electron affinity S', as well as increase the probability of desorption of ions as a result of an increase in the kinetic energy of the excited particles. In this case the effectiveness of the ionization of radicals should depend on the composition of the initial peroxide and the structure of the molecule, on the pathways of its decomposition on the surface, on the composition of intermediate decomposition products, and on their energy state. An elucidation of the peculiarities of ionization of peroxides close in composition but differing in structure and value of the internal energy may give information on the mech- anism of nonequilibrium ionization.

    A static magnetic mass spectrometer with combined ion source, including a source with thermal ionization and a source with electron ionization, was used for the measurements.

    A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 19, No. 2, pp. 214-220, March-April, ]983. Original article submitted May 18, 1982.

    192 0040-5760/83/1902-0192507.50 9 1983 Plenum Publishing Corporation

  • lg I / I o

    -3 -2,O

    I -I,o -45 ~ -2,o -1,s -1,u -45

    Fig. ]. Retention curves of ions (on an emitter of oxidized tungsten at |I00~ i l Cs + ; a) TPA: A) CH~; ,) CH3CO+; E) (CH3)2C0+; x) (CH3)2COCH~;9 CH3C(OCH3)~; b) DPA: A) CH~; ,) CH3CO+; D) (CH~)2CO+; (CH3)2COCH~; O) (CH30)2CCOCH~.

    The possibility of comparing the mass spectra of ionization of peroxides on the surface with the mass spectra of ionization by electrons was thereby achieved. Ionization was brought about by low-energy electrons. Such a comparison is especially important in the investiga- tion of ionization on a surface with the formation of negative ions, since thermal electronic emission of an emitter heated to high temperatures may lead to the formation of negative ions in secondary and tertiary side processes, occurring both on the surface of the emitter and close to it [2]. A scheme of the ion source, the procedure of heat treatment of ion emitters, and the procedure of analysis of the ions according to the energy of translational motion are described in [3].

    The experiments showed that in a stainless steel feeder system, DPA already breaks down intensively at room temperature. Therefore a glass feeder system with capillary inlet of peroxide vapors directly into the zone of the thermoemitter was used. The ampul with the peroxide was usually at a temperature of 273~ the value of the total molecular flux of per- oxide into the ion source was regulated by varying the cross-section of the flow-passage opening in the glass valve.

    Table I presents the mass spectra of the ionization of DPA and TPA on oxidized tungsten. The mass spectra are close in composition but have lines specific for each peroxide (m = 89 amu in the spectrum of TPA; m = 57 and ;]7 amu in the spectrum of DPA). In addition, there is a substantial difference in the relative intensity of the lines of ions of the same com- position, formed from these peroxides. The difference in the composition of the mass spectra of DPA and TPA is associated with the fact that in the reaction zone arising during the de- composition of these molecules, there are different amounts of the reacting components.

    An investigation of the energy state of ions of the same composition, obtained in the decomposition of peroxides under comparable experimental conditions, may promote an e luc ida- tion of the question of whether the ionizing products of primary decomposition of peroxides are in a uniform state of excitation. To clarify this we investigated the distributions of the ions being desorbed according to energy of translational motion, obtained by the methods of retention curves (as in []]); the experimental conditions (temperature of emitter, vapor pressure of peroxide in the instrument) were kept the same in the experiments with DPA and TPA. The analysis was performed according to the energies of ions with m = 15, 43, 58, 73, and 89 amu, obtained in the ionization of TPA and ions with m = 15, 43, 58, 73, and ]17 amu in the ionization of DPA. The retention curves of these ions were compared with the reten- tion curves of Cs + ions, formed in surface ionization on the emitter of cesium ions delivered from a Knudsen cell. The admission of peroxide vapors to the instrument had no effect on the energy distribution of Cs + ions, consequently, the retention curves of Cs + ions in these experiments characterize the thermal equil ibrium desorption of ions.

    The retention curves cited in Fig. |a are evidence that in the ionization of decomposi- tion products of TPA, the ions CH3(OCH3)2 + have the closest to an equil ibrium energy distri- bution. Other ions have a more pronounced nonequil ibrium distribution; this result agrees with that obtained in [ l] . Figure ]b presents one of a series of retention curves of ionized

    193

  • zo (zlz,.o,,) ' J -ll -2

    -,/S r "~ / ' I I I I I

    600 700 800 900 1000 T, lf

    ],o! %5

    0,5

    0 tO00 1500 2000 gtr

    Fig. 2 Fig. 3

    Fig. 2. Temperature dependences of the ionic cur- rents in the ionization of TPA and Cs on oxidized tungsten: Q) Cs+; *) CH3CO+; x) (CH3)=COCH3 +.

    Fig. 3. Dependence of the current of CH3C02- (I), 02- (2) ions and work function of the emitter (3) on the temperature in the ionization of TPA on tung- sten with the formation of negative ions.

    products of the thermal decomposition of DPA, showing that this peroxide also forms excited particles that do not have time to reach thermal equil ibrium with the solid during their lifetime on the surface. In the region of ion-retaining potentials, the functions log I = f(Ua) are well approximated by straight lines. Thus, ions being desorbed are characterized by a quasiequil ibrium distribution according to energy of translational motion, indicating the absence of transmission of the discrete impulse by the particles being desorbed and accumulation of excitation energy on one bond. The temperature of this distribution can be determined according to the slope of the curves.

    The retention curves of excited ions formed in the decomposition of TPA show that there are at least two groups of ions with different quasiequil ibrium temperatures. One group in- cludes the ions (CHa)=COCHa + and CH~ +, the temperature of which is ~500~ greater than the equilibrium value; the other group includes ions CH3C0 + and (CH3)aCO +, for which the excess of the temperature over the equilibrium value is m800~ This excess is maintained when the temperature of the emitter is varied in the range 900-1200~

    The ions CH3C0 +, (CH3)2C0 + and (CH3)2COCH3 +, obtained in the thermal decomposition of DPA, have energy distributions that virtually coincide with the distributions of the same

    TABLE I. Mass Spectra of the Thermal Ionization of DPA and TPA on Oxidized Tungsten at l l00~

    m, amu

    15 28 29 43 45 47 57 58 59 73 74 75 89

    101 117

    Presumed composition of ions

    Relative intensity

    DPA

    cH? C,2H4 + C2H5 chaco+ CHaCHOH+, CHO~ (CHa)2OH+, CHaO ~ CHaCH2CO+ (CHa)~CO + (CHa)2COH+, CHACO+ {cHa).,COClaf (CHa)2CO+, (CHa)aCOH+ CHaCOHOCH+ CHaC(OCHa) ~ CHaCC(OCH3) ? (CHaO) 2CCOCH #

    19 7

    100 4--+-2

    7__+2 28+_.8 36+ 11 79+_+_.9 38 27

    20___ 10

    TPA

    40 5___2 2-+-I I00 4+__2 3-+-2

    11_+_+2 9__+2

    2O-+-5 2--+-I 7

    115-+-29

    3-4-1

    194

  • TABLE 2. Mass Spectra of DPA and TPA in Ionization by Electrons with Energy 15 eV

    m, amu

    15 18 28 29 32 42 43 45 57 58 59 73 74 75

    101

    Presumed composition of ions

    c.? H20-- C2H ~, CO +

    C2H5+ O,j- CH2CO + CH3CO ~- c 3c.o.+, CHO? CH3CH2CO + (CHa)2CO+ (CHa)2COH+, CHACO._, ~ (CHa),_,COCH~ (CH)~,CO~, (CHa)3COH+ CH 33C()HOCH~ CIt~CC(OCHa) d

    Relative inten~i~

    DPA TPA

    2

    7 5

    38 2

    100

    1

    1

    61 16

    1

    1

    4

    17 5 i

    7 3

    100

    2 65

    8

    1

    1

    6 1

    ions formed from TPA; the CH3 + ions receive less excitation -- their temperature exceeds the equilibrium value by %300~ Thus, at least some of the products of thermal decomposition of DPA and TPA are in the same excited state and are evidently formed in similar processes.

    The supplementary kinetic energy received by the ions as a result of thermal decomposi- tion of peroxides facilitates their desorption, which at equal currents of ions and high tem- peratures leads to a displacement of the initial portions of the temperature dependences of the ionic current into the region of lower temperatures relative to the current of Cs ions (Fig. 2). The ionic currents of the products of thermal decomposition of peroxides increase with increasing temperature of the emitter within the entire temperature range in which the work function of the emitter ~ remains unchanged, i.e., up to T = 1200~ This can be ex- plained by the fact that with increasing temperature of the emitter, the lifetime of the par- ticles on its surface decreases. Therefore, a larger number of particles are desorbed with- out having time to relax, i.e., while they are in an excited state, promoting ion formation. Moreover, for particles possessing an ionization potential V' > ~ at the surface, the prob- abil ity of leaving the surface in the ionized state also increases with the temperature W % exp ~(~- - V')/kT]. Since the ionic current increases with increasing T, it can be con- sidered that the increase in the decomposition of radicals undergoing ionization on the sur- face with increasing temperature is not the determining process.

    The admission of ions of decomposition products of peroxides, produced by thermal de- composition of DPA and TPA in a separate reactor, to the mass spectrometer with ion emitter did not lead to the production of measurable ion currents. Consequently, ionization of radi- cals on the emitter when peroxides are admitted is the result of direct interaction of the peroxide molecules with the thermoemitter. Therefore the use of thermal ionization in the mass spectrometric analysis of acetone peroxides has substantial advantages. In electron ionization of mixtures containing thermal decomposition products of peroxides in addition to the peroxides themselves, it is difficult to distinguish lines corresponding to the ioni- zation of the peroxide in the general mass spectrum. Even using low-energy electrons, it is impossible to find any significant differences in the mass spectra of DPA and TPA permitting an unambiguous determination of the presence of each of these peroxides in mixtures (see Table 2).

    Tungsten emitters were investigated in experiments on the formation of negative ions in thermoionization of peroxides. The mass spectra of the negative ions obtained when peroxides were admitted to a source with an emitter heated to high temperatures consist of many lines, which was noted in [1]. An analysis of the ions according to the method proposed in [2], however, showed that most of the ions were formed not on the emitter but in direct proximity

    195

  • TABLE 3. Mass Spectra of Negative Ions Obtained in Capture of Electrons with Energy 0.6 eV by DPA and TPA Molecules

    m t aml l

    17 32 41 45 48 56 57 58 59 73 75 90

    106 !07

    Presumed composition of ions

    Relative intensity

    OH-

    I-tCCO- HCO~- O~-, H:3COOH- (CO)~-, CH~CHCO-

    CH:~CH~CO-, HC~O,2- (CH~)2CO--, H2C20 ]- CH3COO- (CH3);~CO-, CH;3CH~CO, ]" (CH:3),2CHOO- (CH~O) 2CO- (CH3OL)COO- (CH:50)2CHOO -

    DPA

    4

    306

    I

    3 2

    I00

    2

    TPA

    79 39 48

    53 56O 35O

    100 18

    17 37 29

    to it as a result of capture by the peroxide molecules of electrons emitted by the thermo- emitter. Actually, the mass spectra obtained in these experiments proved very close to the mass spectra of dissociative capture of electrons by peroxide molecules at electron energies close to zero. The results of these experiments are cited in Table 3. In addition, the currents of most of the ions changed with changes in the temperature in proportion to the current of thermoelectronic emission of the emitter. On the surface, however, chiefly ions with m = 59 amu were ionized; from the ratio of the isotopic peaks it was established that these ions are acetate ions (CHACO2-). In the ionization of TPA, substantially smaller cur- rents of 02- ions and traces of HCO0- ions were also obtained.

    Figure 3 presents the temperature dependence of the currents of negative ions formed in the ionization of TPA and the work function of the emitter obtained from measurements of the ionic current of thermoelectro...

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