HITEMP, the high-temperature molecular spectroscopic database

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Abstract

A new molecular spectroscopic database for high-temperature modeling of the spectra of molecules in the gas phase is described. This database, called HITEMP, is analogous to the HITRAN database but encompasses many more bands and transitions than HITRAN for the absorbers H2O, CO2, CO, NO, and OH. HITEMP provides users with a powerful tool for a great many applications: astrophysics, planetary and stellar atmospheres, industrial processes, surveillance, non-local thermodynamic equilibrium problems, and investigating molecular interactions, to name a few. The sources and implementation of the spectroscopic parameters incorporated into HITEMP are discussed.

Introduction

In the second half of the twentieth century, the simultaneous development of computers, high-resolution laboratory spectroscopy, and sensitive detectors for field instruments led to the establishment of a computer-readable archive of spectroscopic parameters applicable for atmospheric transmission and radiance calculations [1]. The standard database for this work is the HITRAN database [2], which is periodically updated and expanded. HITRAN has its origins in applications for conditions of the terrestrial atmosphere, particularly temperatures ranging from the surface of the Earth to the stratosphere. As a result, there are many molecular bands and line transitions that would be significant at high temperatures that have not necessarily been considered for the HITRAN archive. In addition, there are numerous molecular bands, especially in the near-IR and visible spectral regions, that are still missing from HITRAN due to the lack of either experimental data or theoretical calculations of these lines.

The HITRAN database, used as input to various high-resolution transmission codes, has been successful for a vast number of applications. The foremost application is remote-sensing of the terrestrial atmosphere from spectrometers aboard satellites, balloons, and ground-based instrumentation. There are also environmental, industrial, surveillance problems, and numerous other applications. Industrial, environmental, and surveillance applications often require a high-temperature spectroscopic database. However, the use of HITRAN (established at a reference temperature of 296 K) is usually deficient when applied to problems where gases are at elevated temperatures. There is also the obvious requirement in astrophysics to characterize stellar, brown dwarfs, and planetary atmospheres. For example, the recent detection [3] of water in extrasolar planet HD189733b relied heavily on the BT2 line list [4], which is used in the present work as explained below. The analysis would not have been possible with HITRAN nor reliable with the earlier version of the high-temperature analog [5] to HITRAN. The need for a high-temperature database embraces, for example, some planetary atmospheres such as possessed by Venus or many exosolar planets. An example of the inability of HITRAN to adequately characterize the Venus night-side atmosphere was given in Pollack et al. [6]. In addition, having a database with excited energy levels satisfies the requirements of some non-local thermodynamic equilibrium (NLTE) problems in the atmosphere. A typical example is the Meinel bands [7] of OH, which require transitions between very high energy levels that would not be necessary in normal radiative-transfer applications.

Early attempts to produce a high-temperature database simply scaled the HITRAN database to estimate the absorption and emission spectra at elevated temperatures. In the database, the intensity1 of a line transition Sjf between lower state i and upper state f as a function of temperature T is given bySif(T)=Sif(Tref)Q(Tref)Q(T)exp(c2Ei/T)exp(c2Ei/Tref)1exp(c2νif/T)1exp(c2νif/Tref),where Tref is the reference temperature of the database (296 K), Q is the total partition sum, Ei is the energy of the lower state (cm−1), and vif is the energy difference between the initial and final state (given as vacuum wavenumber, cm−1, in the database). The constant c2 is the second radiation constant (c2=hc/k=1.43877 cm K). The quantities in Eq. (1) are all provided in the HITRAN compilation. Although the scaling from room temperature to higher temperatures is sufficiently accurate to estimate the line intensity at higher temperature provided an adequate high-temperature partition function is available, the additional spectral lines that are missing from the simulation often lead to a significant underestimation of the source radiance in specific spectral regions.

To address these issues, an analogous database to HITRAN was established and called HITEMP [5] (hereafter called HITEMP1995 to distinguish it from the current effort). This first edition included only the gases H2O, CO2, CO, and OH. The water-vapor line parameters were the result of a calculation using the direct numerical diagonalization (DND) method [8] and were aimed at being sufficient for 1000 K. There was also a calculation for 1500 K, but with a limited dynamic range of intensities. The carbon dioxide parameters were also the result of the same theoretical methodology [8], applicable at 1000 K. The carbon monoxide line list was adapted from the work of Goorvitch [9] that was constructed for a solar atlas. Finally, the hydroxyl line list [10] was added to HITEMP1995 since an extensive list was available in HITRAN itself for NLTE applications.

One desirable feature in creating the HITEMP database (hereafter called HITEMP2010 when referring to the new edition) is to have it consistent with the HITRAN database. That is, it is preferable to have any transitions in common be identical since some simulations might use HITEMP for the source with HITRAN representing the intervening atmospheric path. Having this correlation of lines was simple for CO and OH, where the HITRAN and HITEMP line lists for these gases were generated from the common sources. For CO2, constraints were applied that heavily weighted the molecular constants in the fit to the HITRAN values. However, for H2O the problem was much more complicated since the data in HITRAN consist of many different contributions, both experimental and calculated, and from different sources. Thus common lines found in HITEMP1995 were superseded by their counterpart in HITRAN (the edition of the HITRAN database [11] at that time). This method relies on the quantum identifications of transitions in both databases being consistent, by no means assured, especially for the higher polyads. This method can also introduce discontinuities in the line positions of bands as one migrates to higher rotational values.

With these limitations of the older version, HITEMP1995, in mind, and with new calculations and experiments that have become available, we embarked on a program [12] to substantially update the database, as described in the following sections.

Section snippets

Structure of the database

The format of HITEMP2010 has been maintained to be the same as that of HITRAN. Thus, the HITEMP1995 edition [5] had the 100-character length transition record that was established in 1986 [13]. The current HITRAN edition has increased the length of each transition to 160 characters. Table 1 gives a description of the current list of parameters, definition of units, and format. The databases are in ASCII files and, while following the HITRAN format makes the HITEMP2010 database quite large, it

H2O

For the HITEMP1995 water vapor line list a cutoff in intensities of 3.7×10−27 cm molecule−1 at 1000 K was employed. This cutoff has significantly reduced the capabilities of HITEMP1995 to accurately predict spectra above 1000 K, although it has been fairly accurate for temperatures below and around 1000 K. Taking into account that the potential-energy surface (PES) and dipole moment surface (DMS) used for the generation of HITEMP1995 are now considered outdated, there is an obvious need of creating

Computation of highly excited molecular spectra

HITEMP continues to be an extremely useful database for many applications requiring the prediction of spectral signatures from highly excited vibrational levels, for example at high altitudes where NLTE conditions prevail, in auroral events, or in rocket plume simulations. Under these conditions, vibrational bands are described by their own vibrational temperatures, which deviate from the local temperature. Current NLTE models, such as SAMM2 [71] predict line strengths by calculating the

Conclusion

A new edition of the high-temperature molecular absorption database, HITEMP2010, has been constructed. This edition includes molecular transitions for five species: H2O, CO2, CO, NO, and OH. Table 4 summarizes the spectral coverage of the new edition, and shows a comparison to the standard atmospheric database, HITRAN, in terms of size. The last column presents the dissociation energies of the molecules.

The structure of the database remains the same as the HITRAN database, but it is expected

Acknowledgments

We acknowledge the support of NASA through the Earth Observing System (EOS) program under the Grant no. NAG5-13534 and the Planetary Atmospheres program under Grant no. NNX10AB94G. We also acknowledge the CHEMS (Computation of Highly Excited Molecular Spectra) SBIR project through Spectral Sciences, Inc. RRG acknowledges support of this research by the National Science Foundation through Grant no. ATM-0803135. VIP and SAT acknowledge support by the Russian Fund of Basic Research under the Grant

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