Loss of atomic oxygen in mass spectrometer ion sources

Abstract

A gas beam consisting of a mixture of atomic and molecular oxygen has been directed at the ion source of a mass spectrometer like those used in sounding rockets for determining the neutral composition of the lower thermosphere. The loss of atomic oxygen on mass spectrometer surfaces was evaluated by flagging the beam in several ways and comparing the experimental results with predicted values. The results obtained suggest that in rocket flights using similar instruments the atomic oxygen densities computed assuming no-loss conditions may be low by a factor of 2.5. Studies made using a beam containing tracer indicate that carbon dioxide observed when atomic oxygen enters the source is formed in a reaction involving atomic oxygen from the beam and carbon monoxide from the surfaces bombarded. Although atomic oxygen is one of the mostimportant constituents of the lower thermosphere, there is little agreement as to its absolute abundance. Recently reported values for any given altitude vary by a factor of 5 or. more. Since the loss of atomic oxygen in conventional mass spectrometer ion sources is not known, uncorrected densities are reported and are believed to be low [Hedin and Nier, 1966; Kasprzak e.t al., 1968; Krankowsky et al., 1968; Mauersberger .et al., 1968; Gross et al., 1968; Bitterberg et al., 1970; Hickman. and Nier, 1972]. Densities deduced from optical absorption measurements [Hall et al., 1963, 1965, 1967] tend to be higher, as are those reported from incoherent scatter measurements [Bauer et al., 1970; Cox and Evans, 1970; Giraud et al., 1972]. Von Zahn [1970], in a comparison of atmospheric densities at 150 km as determined by mass spectrometer composit. ion measurements and by satellite drag measurements, concluded that the values could be reconciled ß if, along with several other adjustments, one assumed that the mass spectrometrically found atomic oxygen results were low by a substantial factor, presumably owing to the loss of atomic oxygen in mass spectrometer ion sources. This view seems to be supported by the results of O•er-Copyright (•) 1973 by the American Geopkysical Union. mann and yon Zahn [1971] who, with a heliumcooled mass spectrometer ion source, obtained an appreciably higher value for the n(O)/n(02) ratio at 120 km than had previously been reported. On the other hand, in a direct comparison between density scale heights below 180 km as found by mass spectrometry and by drag measurements with the low-altitude satellites OV1 15 [Champion et al., 1970a] and OV1 16 [Cham.pio• et al., 1970b], Nier [1972] con-

Gas-Surface Interactions in Low-Earth Orbit

“…At its perigee of 400 km, about 5 % of a monolayer of atomic oxygen was adsorbed on the interior walls of the mass spectrometer. This agrees with laboratory experiments which have shown that atomic oxygen adsorbs on many surfaces [7,8,17]. The realization that atomic oxygen reacts chemically within spaceborne instruments has led to improvements in measurements of upper atmospheric composition [20].…”

“…We have learned much about adsorption and energy accommodation from laboratory measurements. However, we cannot apply laboratory measurements directly to surfaces in space because the AC depends on the amount of atomic oxygen adsorbed on the surface as well as other factors: Atomic oxygen, a principal constituent of the thermosphere, adsorbs on nearly any surface [7,8], and specifically on spacecraft materials [9][10][11][12][13][14][15][16][17]. Oxygen also reacts chemically with many spaceborne materials [12][13][14][15][16].…”

Atomic oxygen-metal surface studies as applied to mass spectrometer measurements of upper planetary atmospheres

“…(1) It has been reported to Lake and Nier [1973] that the loss correction factor may be of the order of 2.5. This number is a correction for surface chemistry only.…”

“…The problem of obtaining accurate atomic oxygen number densities in the earth's upper atmosphere by measurements taken with a mass spectrometer is that the atomic oxygen tends to interact with the mass spectrometer ion source. The nature of this interaction is such that the measured atomic oxygen signal tends to be lower than expected [Lake and Nier, 1973]. This reduced signal has been referred to as the 'atomic oxygen loss.…”

Equatorial composition in the 137- to 225-km region from the San Marco 3 Mass Spectrometer

“…faces surrounding or enclosing the mass spectrometer ion source [yon Zahn, 1970;Kasprzak et al, 1968;Lake and Nier, 1973;Niemann et al, 1973]. Offermann and yon Zahn [1971] have solved some of these problems by applying cryogenic pumping techniques in their mass spectrometer.…”