Wilbur S. Hurst

Line interference effects in the vibrational Q-branch spectra of N2 and CO

Self-broadened (≈20–200 kPa) Q-branch spectra are measured by high-resoIution cw-stimulated Raman spectroscopy. Line overlap in these spectra is described using a relaxation matrix formalism and a first-order (in density) solution to the resulting equation is used to fit the data. The parameters of this model are analyzed in terms of rates of rotational energy transfer.

In Situ Raman Spectroscopic Investigation of Aqueous Iron Corrosion at Elevated Temperatures and Pressures

In situ Raman spectroscopy was employed to investigate iron corrosion in air saturated water at a pressure of 25.1 MPa and temperatures from 21 to 537°C. Upon heating, various combinations of Fe 3 O 4 . α-Fe 2 O 3 , γ-FeOOH, and γ-Fe 2 O 3 were observed depending on the location on or temperature of the iron coupon. In some cases, different species were observed at the same temperature at different locations on the surface. This was attributed to oxygen concentration gradients in the solution caused by recirculation zones in the cell. The surface of the corrosion coupon changed little after it was heated to 537°C. This was attributed to the formation of a relatively thick, protective oxide scale after exposure to supercritical water. The ex situ Raman spectra were very similar to the in situ spectra obtained during cooling but different from those obtained during heating. This indicates that the corrosion layer present during cooling is similar to that observed ex situ but different from that observed during heating. Ex situ characterization of the coupon identified a two layered structure: an inner corrosion layer consisting of Fe 3 O 4 and α-Fe 2 O 3 and an outer layer consisting of γ-Fe 2 O 3 and α-Fe 2 O 3 .

Relative Intensity Correction of Raman Spectrometers: NIST SRMs 2241 through 2243 for 785 nm, 532 nm, and 488 nm/514.5 nm Excitation

Standard Reference Materials® SRMs 2241 through 2243 are certified spectroscopic standards intended for the correction of the relative intensity of Raman spectra obtained with instruments employing laser excitation wavelengths of 785 nm, 532 nm, or 488 nm/514.5 nm. These SRMs each consist of an optical glass that emits a broadband luminescence spectrum when illuminated with the Raman excitation laser. The shape of the luminescence spectrum is described by a polynomial expression that relates the relative spectral intensity to the Raman shift with units in wavenumber (cm−1). This polynomial, together with a measurement of the luminescence spectrum of the standard, can be used to determine the spectral intensity-response correction, which is unique to each Raman system. The resulting instrument intensity-response correction may then be used to obtain Raman spectra that are corrected for a number of, but not all, instrument-dependent artifacts. Peak area ratios of the intensity-corrected Raman spectrum of cyclohexane are presented as an example of a methodology to validate the spectral intensity calibration process and to illustrate variations that can occur in this measurement.

Measurement and prediction of Raman Q-branch line self-broadening coefficients for CO from 400 to 1500 K

The J and temperature dependence of the self‐broadening coefficients for the Raman Q‐branch lines of pure CO have been experimentally determined for Q(J) transitions with J=0–38 and for temperatures in the range 400–1500 K. It is shown that a fitting law, based on a modified exponential energy‐gap model for the rates of state‐to‐state rotationally inelastic collisions, can account for the observed J dependence. The two parameters that determine the J dependence are found to be essentially independent of temperature. A temperature scaling function, recently proposed for N2, is added to the basic rate law, and accurate predictions of both the J and the T dependence of these coefficients and those previously reported at 298 K are obtained. This rate law model, used in conjunction with a relaxation matrix description of the Q‐branch spectrum, is shown to give good agreement with the observed, partially collapsed spectrum at 2.8 atm and 295 K.

In Situ Raman Spectroscopic Investigation of Stainless Steel Hydrothermal Corrosion

Abstract In situ Raman spectroscopy was used to investigate corrosion of Type 304L stainless steel ([SS] UNS S30403) in airsaturated water at a pressure of 25.2 MPa and temperatures up to 496°C in an optically accessible flow cell. The exposed Type 304L SS coupon also was characterized ex situ with Raman spectroscopy and scanning electron microscopy. After exposure, nickel(II) iron(III) oxide (NiFe2O4) and alpha chromium(III) oxide hydroxide (α-CrOOH) phases were detected on the coupon surface. The NiFe2O4 phase was first identified after the coupon was heated to 247°C and was observed under all subsequent conditions. Upon heating from 247°C to 326°C, the crystallinity and possibly the thickness of the NiFe2O4 phase increased. The α-CrOOH phase was first identified after the coupon was heated to 326°C and was observed under all subsequent conditions. At least a portion of the α-CrOOH phase probably originated as a corrosion product released from the optical cell and/or flow system. In situ Raman spectra i...

COMPARISON OF ROTATIONAL RELAXATION RATE LAWS TO CHARACTERIZE THE RAMAN Q-BRANCH SPECTRUM OF CO AT 295 K

Abstract We test the ability of the energy corrected sudden (ECS), modified exponential gap (MEG) and the statistical power-exponential gap (SPEG) rate laws to characterize line broadening and line interference in the CO Q-branch at 295 K. All three rate laws fit the experimental linewidth data. The ECS law is found to predict too much spectral collapse. The MEG and SPEG laws both adequately model spectral collapse, but with different implications about the role of dipolar and quadrupolar symmetry forces in CO:CO line broadening. From semiclassical calculations of CO linewidths, we conclude that the SPEG law with a restriction to even ΔJ changes is the more physically correct model.

Measurement and rate law analysis of D2Q‐branch line broadening coefficients for collisions with D2, He, Ar, H2, and CH4

Continuous‐wave stimulated Raman spectroscopy has been used to obtain high resolution vibrational Q‐branch spectra at room temperature for pure D2 and D2:He, D2:H2, D2:Ar, and D2:CH4 mixtures. Measurements have been made for J=0–5 in the density region of 0.5–20.0 amagat, from which line broadening coefficients have been determined. These coefficients have been analyzed using a modified exponential energy gap rate law to identify the contributions of rotationally inelastic collisions and vibrational dephasing collisions to the linewidth. This analysis has assumed that vibrational dephasing is independent of rotational state, in accord with available theoretical studies. Results are compared with experimental and theoretical work on H2, HD, and D2, thereby characterizing the contributions of rotationally inelastic and vibrational dephasing collisions to the line broadening coefficients as a function of both rotational level and collision partner.

Rotational collisional narrowing in the NO fundamental Q branch, studied with CW stimulated Raman spectroscopy

Self‐broadened NO Q‐branch spectra were obtained in the pressure region ∼20–100 kPa. We determined J‐ and Ω‐dependent pressure broadening coefficients. The observed collisional narrowing was fitted by means of a relaxation matrix theory, incorporating recent experimental and theoretical values of NO state‐to‐state rates. A ‘‘fitting law’’ representation of the state‐to‐state rates yielded good agreement with both the measured broadening coefficients and the observed spectrum.

In situ Raman spectroscopic investigation of zirconium-niobium alloy corrosion under hydrothermal conditions

Abstract In situ Raman spectroscopy was employed to investigate corrosion of a zirconium–niobium alloy in air-saturated water at a pressure of 15.5 MPa and temperatures ranging from 22 to 407 °C in an optically accessible flow cell. Monoclinic ZrO 2 (m-ZrO 2 ) was identified under all conditions after the coupon was heated to 255 °C for 19 h. Cubic ZrO 2 (c-ZrO 2 ) was tentatively identified in situ during heating at temperatures between 306 and 407 °C, but was not observed under any other conditions. Species tentatively identified as α-CrOOH and a Cr VI and/or Cr III /Cr VI compound were observed in situ during heating at temperatures between 255 and 407 °C, but were not observed under any other conditions. The chromium compounds were identified as corrosion products released from the optical cell and/or flow system.

In Situ Raman Spectroscopic Investigation of Nickel Hydrothermal Corrosion

Abstract A nickel coupon was exposed to air-saturated water at a pressure of 25.4 MPa and temperatures ranging from 21°C to 460°C in an optically accessible flow cell. In situ Raman spectra were collected at a number of temperatures as the coupon was heated and then cooled. The coupon also was characterized ex situ with Raman spectroscopy, scanning electron microscopy, energy dispersive x-ray spectrometry, and x-ray diffraction. Corrosion species were observed in situ at temperatures >249°C during heating and under all conditions during cooling. The species were identified as nickel monoxide (NiO) and alpha chromium (III) oxide hydroxide (α-CrOOH). The α-CrOOH originated as a corrosion product released from the optical cell and/or flow system. The NiO Raman features were more intense during cooling than at the corresponding temperatures during heating, indicating that NiO was present in greater amounts and/or was more crystalline during cooling.