Mass Spectrometric Study of Subliming Selenium

Abstract

In the mass spectrum of the vapor from grey selenium subliming in the temperat~ue range 102 to 187 "C all possible Sen+ ions between n = 1 and 12 = 8 were detected in measurable quantities. The ions Seab, Seec, Se,+, and SesLarise mainly from ionization of the corresponding parent molecules, which are believed to be clclic. .It 175 "C the composition of the vapor is Se;, ,< 29.0%, See >, 57.57;, Se, >, 11.4%, and Ses >, 2.0%. Thermodynamic data are reported for the sublimation of these molecules. Small amounts of See+ and Selot were also detected.In the condensed state, selenium can exist in a \\:ide variety of different forms. The thermodynamically stable forin a t tempel-atures up to the melting point is grey, metallic, or hexagonal selenium (1, 2). In this variety the selenium atoins forin spiral chains (3, 4) in which adjoining atoms form normal single bonds.Red crystalline selenium is inonoclinic and two forins have been found (6, 6). The form is composed of Sea rings (7). According to Burbanli (7) Vitreous selenium is formed by rapid cooling of inolten selenium. Altl~ough various views on its structure have been proposed (12-18) i t seeins certain that the chain structure found in grey seleniuin exists in this variety but the chains have become enmeshed and intertwined. When it is dissolved in iodine the average length of the selenium chain is about 100 atoms (19). Vitreous seleni~~in transforms to grey selenium, taliing 20 days a t 43", 72 h a t 55", 12 h a t 70" (lo), and 1 h a t 100 O C (20).Red amorphous seleili~lm also contains chains of selenium atoins (21), the spirals being slightly defornled and randomly placed along the axis of the chain (22). Berthelot (23) distinguishes t\\-o varieties of amorphous selenium.Contradicting values have been given for the density of selenium vapor (34-29). I-Iolvever, all results indicate progressive dissociation into sn~aller nlolecules as the temperature rises.Vapor pressures have been measured by several methods (29-3s). Determinations involving the Knudsen technique involve a lino~\~ledge of the vapor composition and ~vill be in error if the incorrect assumption is made. I t has been proposed (30) that below 530 O C the vapor consists of Sea and between 550 and SO0 "C the vapor consists of SeG and Sez in equilibrium. AIetzger (39) has found that the average molecular weight of freely evaporating selenium is consistent with the formula Se5 ,, 6. 0 1 1 the basis of electron diffraction studies on selenium vapor a t Ion-pressures, I-1on.e and Lark-I-Iorovitz (40) propose Ses

Исследование молекулярного пучка Se масс-спектрометрическим методом с электронной ионизацией

“…Содержание различных фрагментов селена (X n ) изучалось в рабо-тах [10][11][12][13][14][15][16]. Масс-спектрометрическим методом изме-рена зависимость X n = f (T ) в диапазоне температур 450−1020 K. На нижней границе этого диапазона основ-ным фрагментом является Se 6 , а на верхней -Se 2 , т. е. при этих температурах интенсивно идет процесс терми-ческой деструкции тяжелых молекул Se n .…”

“…Исследования термодинамических особенностей се-лена в широком интервале температур проводилось во многих работах [10][11][12][13][14][15][16][24][25][26], в которых показа-но, что мольный состав паров и процесс деструкции сильно зависят от температуры испарения вещества и существенно влияют на количество атомов в класте-рах селена. Наибольшее число работ по исследованию температурных зависимостей образования ионных кла-стеров селена выполнено для температур выше 500 K, что касается более низких температур, то таких работ значительно меньше.…”

“…2 показаны зависимости темпе-ратурных коэффициентов для наиболее интенсивных пиков в масс-спектре, которые соответствуют ионам Se + (m/z = 80) и Se + 2 (m/z = 160). 1 Приведенные нами зависимости для температурных коэффициентов ана-логичны полученным в [13], где сделано заключе-ние о минимальном температурном коэффициенте для ионов Se …”

Processing of CuInSe2-Based Solar Cells: Characterization of Deposition Processes in Terms of Chemical Reaction Analyses. Final Report, 6 May 1995 - 31 December 1998

“…However, a QMS study of the ion-energy distribution from subliming selenium [204] shows that the species Se5, Se6, and Se7 dominate the vapor flux therefrom.…”

“…This problem is unavoidable in the selenium molecular system since the appearance potential for positively charged ionic selenium molecules varies between 8.3 and 10.4 eV [206] and significant cross-sections for electron dissociative attachment (forming negative ions) extend to above 7 eV for Sen when n=1,2, or 3 [207]. Even assuming prior knowledge of the dissociation pathways and probabilities for each molecular species, inversion of the measured ion currents to extract their parent molecules' distribution is not possible without data for the Se 7 + and Se 8 + peaks, whose most common masses are beyond the range of the QMS, at 554 and 634 AMU, respectively [204]. Keeping these limitations in mind, the selenium thermal cracker QMS characterization results shown in Table 4-1 nevertheless admit to a qualitative interpretation that indicates a significant increase in the fluxes of the lower mass species (particularly Se2) at higher cracker temperature, as displayed in Figure 4-1.…”