T.S. Lakshmi Narasimhan

Appearance potential and electron impact ionisation cross-section of C60

A Knudsen cell mass spectrometric technique was employed to determine the electron impact ionisation cross-section of C60. Ion intensities were measured as a function of time, and weight loss due to evaporation was determined. Similar experiments were carried out with silver to obtain calibration factors. From the weight loss and the ion intensity integrated over the entire duration of the experiment for both siver and C60, the ratio of the ionisation cross-section of C60 to that of silver was obtained. Taking the value for silver from the literature, the ionisation cross-section of C60 at 38 eV was calculated to be (53.5 ± 5.6) × 10−16 cm2. This experimental value (determined for the first time) is discussed in relation to those derived by using various empirical formulae generally applied for obtaining molecular cross-sections. Ion intensities were measured as a function of electron energy to obtain ionisation efficiency curves for C60, and its appearance potential was determined by linear extrapolation of these curves

Homogeneity ranges and thermodynamic properties of the Te-rich phases in the CrTe system

The Te-rich region of the CrTe system was investigated by using high temperature mass spectrometry. The composition range from ∼60 to 82.7 at.% Te was covered by preferentially evaporating off tellurium at T < 700 K from the alloys of starting compositions 66.9, 70.1, 80.0 and 82.7 at.% Te. Evidence for the existence of a new polytelluride (77.0 ± 0.3 to 78.3 ± 0.2 at.% Te) was obtained while the existing uncertainty about the homogeneity range of CrTe3, hitherto considered as the most Te-rich phase in the CrTe system, was removed. This phase exists from 70.5 ± 0.8 to 74.4 ± 0.5 at.% Te. In the case of Cr5Te8, only the Te-rich boundary (63.6 ± 0.6 at.% Te) could be deduced. The activities of tellurium across the single-phase regions were obtained from the p(Te2) measured as a function of composition and those of chromium computed by a Gibbs-Duhem integration. Subsequently Gibbs free energies of formation, ΔfGmo at T = 650 K were also deduced. The values (in kJ mol−1, given in brackets) for the formulae (given in parentheses) corresponding to the Te-rich and/or Cr-rich boundary compositions are: CrTe4−y phase: − [90.7 ± 0.4] (CrTe3.61) and − [90.5 ± 0.4] (CrTe3.35); CrTe3 phase: − [90.4 ± 1.0] (CrTe2.91) and − [89.5 ± 1.5] (CrTe2.39); Cr5Te8 phase: − [86.2 ± 2.2] (CrTe1.75, the Te-rich boundary).

Vapour Pressure and Enthalpy of Sublimation of C70

Abstract The vapour pressure of pure C70 was measured in the temperature range 650–850 K by using the Knudsen effusion mass spectrometry resulting in the equation log(p/Pa) = −10219±78 / T(K) ± 11.596±0.065. The second law enthalpy of sublimation of C70 was determined to be 195.7±1.1 kJ/mol.

Investigation of the vaporization of boric acid by transpiration thermogravimetry and knudsen effusion mass spectrometry.

The vaporization of H3BO3(s) was studied by using a commercial thermogravimetric apparatus and a Knudsen effusion mass spectrometer. The thermogravimetric measurements involved use of argon as the carrier gas for vapor transport and derivation of vapor pressures of H3BO3(g) in the temperature range 315-352 K through many flow dependence and temperature dependence runs. The vapor pressures as well as the enthalpy of sublimation obtained in this study represent the first results from measurements at low temperatures that are in accord with the previously reported near-classical transpiration measurements (by Stackelberg et al. 70 years ago) at higher temperatures (382-413 K with steam as the carrier gas). The KEMS measurements performed for the first time on boric acid showed H3BO3(g) as the principal vapor species with no meaningful information discernible on H2O(g) though. The thermodynamic parameters, both p(H3BO3) and Delta sub H degrees m(H3BO3,g), deduced from KEMS results in the temperature range 295-342 K are in excellent agreement with the transpiration results lending further credibility to the latter. All this information points toward congruent vaporization at the H3BO3 composition in the H2O-B2O3 binary system. The vapor pressures obtained from transpiration (this study and that of Stackelberg et al.) as well as from KEMS measurements are combined to recommend the following: log [p(H3BO3)/Pa]=-(5199+/-74)/(T/K)+(15.65+/-0.23), valid for T=295-413 K; and Delta sub H degrees m=98.3+/-9.5 kJ mol (-1) at T=298 K for H3BO3(s)=H3BO3(g).

Vapor Pressure Measurements by Mass Loss Transpiration Method with a Thermogravimetric Apparatus

Thermobalances are used for equilibrium vapor pressure measurements based on both effusion and transpiration methods. In the case of the transpiration method, however, despite the numerous advantages a thermogravimetric apparatus can offer, it is not as widely used as is the conventional apparatus. In this paper, the difference that can exist in the vapor phase compositions in an effusion cell and in a transpiration cell is shown first with two examples. Subsequently, how a commercial thermobalance was utilized to perform transpiration experiments that conform to the basic principle of the transpiration method and yield vapor pressures consistent with the Knudsen effusion mass spectrometric method is described. The three systems investigated are CsI(s), TeO(2)(s), and Te(s), each known to vaporize congruently, but in different manner. A critical analysis was performed on the information available in the literature on transpiration measurements using thermogravimetric apparatuses, and the salient findings are discussed. Smaller plateau regions than with conventional transpiration apparatuses and the lack of evidence for perfect transpiration conditions in some transpiration thermogravimetric investigations are shown with a few examples. A recommendation is made for the use of the rate of mass loss versus flow rate plot to ascertain that the usual apparent vapor pressure versus flow rate plot corresponds to a meaningful transpiration experiment.

Vaporisation studies on tellurium dioxide: A Knudsen effusion mass spectrometric study

Abstract The vaporisation of TeO 2 (s) was studied by Knudsen effusion mass spectrometry. The vapour phase was found to consist of (TeO 2 ) n (n = 1−3)(g), (TeO) n (g) (n = 1−3) and Te 2 (g). The p − T relations of TeO 2 (g), (TeO 2 ) 2 (g) and (TeO 2 ) 3 (g) were derived to be log( p /Pa) = (−13534 ± 78)/ T + (14.241 ± 0.09) (750–950 K), log( p /Pa) = (−14823 ± 212)/T + (14.373 ± 0.242) (825–950 K) and log( p /Pa) = (−19074 ± 540)/ T + (17.337 ± 0.606) (850–921 K) respectively. From the partial pressures, Δ r H 298.15 0 of n TeO 2 (s) = (TeO 2 ) n (g) ( n = 1–3) were evaluated by second and third law methods. Also, enthalpy of the pressure independent reaction TeO 2 (s) + TeO 2 (g) = (TeO 2 ) 2 (g) has been evaluated. Using the Δ f H 298.15 0 of TeO 2 (s), Δ f H 298.15 0 of (Te0 2 ) n (g) ( n = 1–3) were calculated. The partial pressure and enthalpy data for (TeO 2 ) 3 (g) have been obtained for the first time.

A mass spectrometric study of the dissociation of the gaseous diatomic tellurium molecule

Abstract The dissociation equilibrium, Te 2 (g) = 2Te(g), was evaluated from the partial pressures of Te(g) and Te 2 (g) over different metal-tellurides (where the metal = Fe, Cr, Ni or Mo). The equilibrium constants, deduced as a function of temperature (820–1285 K), correspond to a range of monomer-to-dimer ratios, i.e. p (Te)/ p (Te 2 ) varying from ~ 0.02 to 1. They are consistent with the available results corresponding to higher temperatures (1195–1467 K), and therefore, the two sets of data were combined into the following dissociation constant-temperature relation (820–1467 K): The resulting third-law dissociation enthalpy of Te 2 (g), 257.6 ± 4.1 kJ mol −1 , which we recommend, is in excellent agreement with photoionization or spectroscopie results. The pressure-independent reaction, Te(g) + Te (in condensed phase) = Te 2 (g) was also evaluated and the following relation was obtained: This evaluation confirmed the mutual consistency of the p (Te) and p (Te 2 ) values employed to deduce the dissociation constant-temperature relation as well as the dissociation energy reported here. Furthermore, a correlation between the monomer-to-dimer ratio in the vapour phase to the activity of tellurium in the condensed phase is brought out.

Appearance energy and electron impact ionisation cross-section of C70

Abstract The electron impact ionisation cross-section of the C 70 molecule as well as the appearance potential of the C + 70 ion were determined by high temperature mass spectrometry. A molecular beam of C 70 effusing from a Knudsen cell was ionised by a beam of 38 eV electrons. The appearance energy was determined by linear extrapolation of the ionisation efficiency curves. By comparing the integral of the ion intensity vs. time curve and the corresponding weight loss from the Knudsen cell for C 70 and silver, it was possible to deduce the ionisation cross-section of C 70 as (54.5 ± 1.5) × 10 −16 cm 2 . This experimental value is discussed in comparison with those derived using various empirical formulae. The ionisation cross-section of C 70 is very nearly the same as that for C 60 .

Electron impact ionisation of fullereness: appearance energies of C+60, C2+60 and C3+60

Abstract The ionisation efficiency curves for the production of singly, doubly and triply charged ions from C60 by electron impact are reported for the first time up to an electron energy of 80 eV. Appearance energies were derived by linear extrapolation of ionisation efficiency curves and found to be 8.1 ± 0.5, 18.4 ± 0.5 and 33.2 ± 1.0 eV respectively for C+60, C2+60 and C3+60. The shape of the ionisation efficiency curves suggests the participation of several excited states in the ionisation process. Relative abundances of the three ions at various electron energies are also presented.

A ternary phase diagram of the Mn-Te-O system at 950 K

A ternary phase diagram of the Mn-Te-O system at 950 K has been established in the composition range in and around the MnO-TeO 2 pseudo binary line. Various preparation methods were employed to confirm the co-existence of different ternary phases. The results of these phase equilibration studies were revalidated by the invariancy of partial pressures at constant temperature during high temperature mass spectrometric vaporization experiments. The following three-phase regions have been identified: MnO + Mn3O4 + Mn6Te 5O16 (phase region 1; PH1), Mn3O4 + Mn6Te 5O16 + MnTeO3 (phase region 2; PH2), Mn3O4 + MnTeO3 + Mn3TeO6 (phase region 3; PH3), and MnTeO3 + Mn2Te 3O8 + Mn3TeO6 (phase region 4; PH4). The complex nature of the Mn-Te-O ternary system was revealed by the interesting results obtained by us with regard to preparation of samples and mass spectrometric vaporization experiments.