Calibration method at the N K-edge using interstitial nitrogen gas in solid-state nitrogen-containing inorganic compounds

The standard method of soft X-ray beamline calibration at the N K-edge uses the ν = 0 peak transition of gas-phase N₂. Interstitial N₂ gas trapped or formed within widely available solid-state ammonium- and amine-containing salts can be used for this purpose, bypassing gas-phase measurements. Evidence from non-nitrogen-containing compounds (KH₂PO₄) and from He-purged ammonium salts suggest that production of N₂ gas is through beam-induced decomposition. Compounds with nitrate or nitrite as anions produce coincident features and are not suitable for this calibration method.

1. Introduction

The ideal method for energy calibration of soft X-ray analysis at the nitrogen K-edge is the measurement of the vibrational peak positions of gas-phase N₂ with an in-line mounted gas cell. The vibrational manifold of the 1s → π* transition is well known from electron spectroscopy, and the ν = 0 peak has an energy of 400.8 eV (Sodhi & Brion, 1984; Schwarzkopf et al., 1999). Not all beamlines have facilities for gas-phase measurements, however, and in beam-time shifts where solid-state measurements are planned, set-up and use of the gas cell can be unwieldy and time-consuming.

Characteristic N₂ molecular vibrations in nitrogen-containing salts were observed during an investigation of natural reference compounds (Leinweber et al., 2007). Also, trapped N₂ gas has been reported in the characterization of N₂⁺-bombarded alumina (Holgado et al., 2003), and of compound semiconductors (Petravic et al., 2006; Ruck et al., 2004). The objective of this study was to conduct an assessment of suitable inorganic compounds that can be used for a simple alternative calibration method at the N K-edge using high-resolution scans over energies corresponding to N₂ gas.

2. Materials and methods

2.1. Reference compounds

The inorganic N-containing salts used as reference compounds are listed in Table 1. All reference compounds were pulverized with a mortar and pestle, affixed to double-sided conductive carbon tape (SGE, Toronto, Ontario, Canada) and mounted onto stainless steel sample discs. In addition, hydroxylamine hydrochloride and ammonium sulfate were dissolved in deionized water, purged with ultra-high-purity helium, evaporated under helium to remove existing N₂ gas, and pulverized and mounted as described above. Gas-phase photoabsorption N₂ measurements were obtained using a double ionization gas cell with a 10 cm active pathlength in each chamber (Yates et al., 2000) for comparison with N₂ vibrations measured in solid-state samples.

Table 1
Names, formulae, CAS (Chemical Abstracts Service) numbers and suppliers for compounds used in this study

Compound	Formula	CAS number	Supplier
Ammonium chloride	NH4Cl	12125-02-9	AnalaR
Ammonium phosphate, dibasic	(NH4)2HPO4	7783-28-0	BDH
Ammonium phosphate, monobasic	(NH4)H2PO4	7722-76-1	EMD
Ammonium sulfate	(NH4)2SO4	7783-20-2	Fisher
Ammonium nitrate	NH4NO3	6484-52-2	Sigma-Aldrich
Hydroxylamine hydrochloride	NH2OH.HCl	5470-11-1	Chemservice
Potassium nitrate	KNO3	7757-79-1	EMD
Sodium nitrite	NaNO2	7632-00-0	Sigma-Aldrich
Potassium dihydrogen phosphate†	KH2PO4	7778-77-0	Baker

†Ultra-high purity (99.8%).

2.2. N K-edge XANES

Nitrogen K-edge X-ray absorption near-edge structure (XANES) spectra were collected using the spherical grating monochromator beamline 11ID-1 at the Canadian Light Source, Saskatoon, Saskatchewan, Canada. This facility delivers 10¹¹ photons s⁻¹ at the N K-edge with a resolving power (E/ΔE) greater than 10000 (Regier, Krochak et al., 2007; Regier, Paulsen et al., 2007). Fluorescence yield (FLY) data were recorded using a two-stage multichannel plate detector that was operated in parallel with total electron yield (TEY) data collection through measurement of the drain current from the sample. High-resolution scans were taken using a step size of 0.01 eV over the expected range for N₂ gas, and of 0.1 eV for the remainder of the scan. Spectra were normalized to the incident flux using an in-line Au mesh which was refreshed by evaporation in situ prior to data collection. Data processing was carried out using the Athena software package (version 0.8.050). Data from at least two scans were averaged, and background-corrected by a linear regression fit through the pre-edge region followed by normalization to an edge step of unity.

3. Results and discussion

Fig. 1 shows FLY data of N K-edge XANES scans for the inorganic N-containing compounds studied, with panel (a) illustrating the energy region surrounding the vibronic structure of N₂ gas in detail, and panel (b) showing the full scan. Nitrogen gas vibronic structure is clearly evident in the reduced N-containing (i.e. ammonium- or amine-containing) salts that do not have an oxidized N anion. The ν = 0 transition occurs at the same peak position in the solid phase as it does in the gas phase (Fig. 1a). The data of Chen & Sette (1989) show no significant shift in the peak position until the peak width broadens by over 100 meV, by which time a significant change in the overall shape has occurred. Provided there is a valley between the second and third peaks, we quantitatively estimate the accuracy to be within 100 meV. Peak widths, however, vary owing to lifetime reduction through solid-state mechanisms. This change correlates with the crystal lattice constant, and thus with confinement of N₂ gas within the solid phase. This has been discussed in detail with respect to compound semiconductors by Petravic et al. (2006).

Figure 1 Nitrogen K-edge FLY spectra showing N2 gas in the solid state of inorganic salts used in this study. (a) Detail of the high-resolution section, (b) full N K-edge scan. The dotted line corresponds to 400.8 eV, which is the ν = 0 peak for the vibrational manifold of the 1s → π* transition for N2 gas (Sodhi & Brion, 1984; Schwarzkopf et al., 1999).

The presence of nitrate (NO₃) or nitrite (NO₂) produces a feature at 401.7 eV, which is coincident with 1s → π* of N₂ gas, rendering these compounds less useful for calibration purposes. Although observed previously (Leinweber et al., 2007), this feature remains unidentified. It should be noted, however, that this feature was not observed in previously published analyses of KNO₃ (Vinogradov & Akimov, 1998; Rodrigues et al., 2007), suggesting that it may be a product of beam-induced decomposition. If this is the case, it is not likely to be decomposing to nitrous oxide (N₂O) or nitrogen dioxide (NO₂), as this feature does not match previously published spectra for these oxides of N (Gejo et al., 2003; Adachi & Kosugi, 1995). Decomposition to NO, however, is observed here in NH₄NO₃, with vibrations corresponding to published results (Adachi & Kosugi, 1995; Yates et al., 2000; Remmers et al., 1993).

Certainly, atmospheric N₂ trapped within the solid phase would explain the occurrence of this feature, implying that the use of any inorganic salt would suffice as a calibration material. However, spectra obtained for N-free ultrapure potassium phosphate (dibasic) showed no evidence of N₂ gas vibrational structure (Fig. 1). In addition, we analyzed ammonium sulfate and hydroxylamine hydrochloride that had been dissolved, purged of N₂ and recrystallized under an ultra-high-purity helium atmosphere. These scans showed the presence of N₂ gas, matching those obtained for the unprocessed salts (data not shown). While it is beyond the scope of this paper to determine the mechanism of entrapment or production of interstitial N₂, these results suggest that it is derived from decomposition of the ammonium moiety. Indeed, for calibration purposes, the salt must contain ammonium- or amine-N to be of use.

TEY spectra for selected scans are shown in Fig. 2, with panel (a) illustrating the region encompassing N₂ gas within the full scan pictured in panel (b). Scans for all the ammonium-containing salts, except NH₄NO₃, showed similar characteristics; thus only data for (NH₄)₂SO₄ are presented here as a representative scan. The N₂ gas vibronic feature is suppressed in all TEY scans for all substances, and is absent from NH₄NO₃ and KNO₃. TEY (with an estimated penetration depth of <10 nm) is more surface sensitive than FLY (with an estimated penetration depth of 70–100 nm) (Frazer et al., 2003; Katsikini et al., 1997). This suggests that the N₂ gas originates (i.e. is entrapped or produced) below the surface layer, and that any N₂ at/near the surface is removed in the vacuum chamber. Therefore, as a calibration method, TEY provides a less useful measure of absorption when compared with FLY. TEY data also revealed a stronger feature at 401.7 eV seen in KNO₃ and NaNO₂, indicating that, if this is a decomposition product, the mechanism of its formation is surface-sensitive.

Figure 2 Nitrogen K-edge TEY spectra showing suppressed N2 gas detection. (a) Detail of the high-resolution section, (b) full N K-edge scan. The spectrum for ammonium sulfate is representative of all spectra of ammonium- and amine-containing salts.

4. Conclusions

Interstitial N₂ gas contained within solid-state inorganic ammonium- and amine-containing salts can be used to quickly and accurately calibrate soft X-ray beamlines at the N K-edge without the need for gas-cell measurements. Dinitrogen gas contained within these solids may be generated via beam-induced decomposition. Other, as yet unknown, compounds may be produced if the anion is an oxidized N moiety such as nitrate or nitrite. Characteristic N₂ molecular features are observed more strongly with FLY, indicating formation/presence within the bulk material.