The rotationally resolved infrared spectrum of TiO and its isotopologues
Graphical abstract
Introduction
Since 1904, when the British astronomer Alfred Fowler (1868–1940) showed similarities between the spectra of Antarian stars1 and the arc spectrum of titanium monoxide (TiO), TiO has become a molecule of astrophysical relevance [16]. In the second half of the 20th century, the blue-green emission in late-type stars were assigned to electronic transitions of TiO by Merrill et al. [33]. Since 1973, the strength of the prominent VIS-UV lines of TiO in spectra of late-type stars are used to classify stars within the Morgan-Keenan spectral classification scheme [34]. In rare cases, emission bands of TiO can be found in optical spectra of warm circumstellar environments [4], [19]. Metal oxides, like TiO, are thought to play an important role in the change of the VIS-UV apparent magnitude in Mira-type variable stars at optical as well as infrared wavelengths, as has been demonstrated by Reid and Goldston [50].
Up to now, TiO signatures in stellar spectra are the topic of many observational studies. Chavez and Lambert [9] detected isotopologues of TiO towards local M-dwarf stars, such as GJ699 or GJ701, at optical wavelengths. The derived isotopic abundances in these objects are similar to the natural abundance found on earth. Kamiński et al. [20] detected pure rotational transitions from TiO towards the oxygen-rich late-type star VY Canis Majoris for the first time using the submillimeter array (SMA) between 279.1 GHz and 335.1 GHz. In a follow-up study towards Mira (o-Ceti) at submm-wavelength, the detection of all stable titanium isotopologues was reported, with the exception of 47TiO [21]. In these studies it was shown, that the titanium-bearing molecules, TiO and TiO2, are found outside the dust forming regions. From the high abundance, the authors concluded that a substantial fraction of titanium is present as gas-phase species and not as solid dust grains. This is an indication that titanium oxides do not initiate the dust formation - as previously believed. The authors added that titanium might still support the formation of silicate dust. Recently, pure rotational transitions of vibrationally excited TiO were identified towards the AGB stars R Dor and IK Tau by Danilovich et al. [10].
First evidence for signs of TiO in atmospheres of extrasolar planets (exoplanets) were found in the atmosphere of the ultra-hot jupiter WASP-121b [14], while earlier studies indicated no presence of TiO in the atmosphere of other investigated exoplanets [18], [54]. TiO is proposed to contribute to thermal inversion in the atmospheres of hot Jupiters, which is in good agreement with self-consistent atmospheric models Piette et al. [46]. Recently, an improved TiO line list was added to the ExoMol database [56], [31], which contains molecules associated with atmospheres of exoplanets. Based on this line list, Pavlenko et al. [41] investigated the spectra of the M dwarfs GJ15A and GJ15B, revealing non-solar isotopic titanium ratios deduced from synthetic spectra of the TiO isotopologues. Using the Spitzer (IR) Space Telescope, Smolders et al. [55] published a paper, where they assigned an infrared emission band of the S-type star NP Aurigae to TiO. The spectrum was taken at a resolution of 2 cm−1 and no individual ro-vibrational transitions could be resolved. In the same work, further candidates of TiO emission bands were suggested in the spectra of RX Psc and V899 Aql, but could not be verified.
In terms of laboratory investigations, TiO has been intensivly studied in various experiments. An overview of the works targeting the main isotopologue 48Ti16O is given by McKemmish et al. [32]. In the early stages, TiO was commonly produced from modulated high-voltage arc discharge sources, like in the work of Phillips [43]. J. Phillips identified the 1 – 1 and the 1 – 1 bands of the main isotopologue of TiO in the spectra using a grating spectrograph. Initially, it was mistakenly assumed that the triplet electronic state 3 is the ground-state [30], [44]. In 1969, the 3 state was correctly assigned as electronic ground-state [45]. Fletcher et al. [15] investigated the hyperfine splitting of 47TiO, that originates from the spin of the rare titanium isotope, using laser induced fluorescence spectra from the 3 – 3 transition. Amiot et al. [1], [2] performed crossed beam experiments between a molecular beam of TiO and a continuous wave tunable laser to study the 3 – 3 and the 1 – 1 bands at sub-Doppler resolution. The laser induced fluorescence spectra led to a comprehensive determination of the rotational molecular constants, as well as spin–orbit- and spin–spin-coupling constants, A and . A few years later, two bands, 3 – 3 and 1 – 1 of the TiO main isotopologue, were identified in sunspots by Ram et al. [47], [48]. This observation was based on laboratory measurements conducted between 10,000 and 16,000 cm−1, where TiO was produced using a hollow cathode lamp. The same authors also improved the 3 ground-state rotational constants of 48TiO by including pure rotational transitions from jet cooled-measurements. Namiki and Ito [37] and Namiki et al. [38], [39] analyzed several vibronic transitions of 48TiO of the 3 electronic ground-state in the optical frequency range. Measurements of the 3 – 3 transition of all TiO isotopologues in a jet-cooled experiment were performed by Kobayashi et al. [24]. Pure rotational transitions of laser ablated TiO were measured by Kania et al. [22] in a supersonic-jet expansion utilizing millimeter-wave spectroscopy. Recently, Lincowski et al. [29] presented a spectroscopic analysis of the rare isotopologues of TiO by means of high-resolution millimeter/submillimeter spectroscopy. From this study hyperfine constants of 47TiO and 49TiO were determined. The results agree well with those obtained by Fletcher et al. [15]. Lincowski et al. [29] also obtained rotational constants, including centrifugal distortion constants for spin–spin and spin–orbit parameters, for all stable titanium isotopologues. Breier et al. [7] conducted measurements for all stable titanium isotopologues of TiO around 300 GHz. The data were analyzed using a mass-independent Dunham approach, which allows to investigate all isotopologues in a global fit, thus combining measurements at microwave and optical wavelengths. In turn, the Dunham analysis allowed to predict frequencies for the radioactive molecule 44TiO within a sub-MHz uncertainty. This unstable isotopologue decays with a half-life of 60 years and is of particular interest in the context of young supernova remnants [3], [53], [57]. The titanium isotope 44Ti is synthesized in core-collapse supernovae during helium burning reactions.
In this study, we present the first measurement of the infrared spectra of TiO and of all stable titanium isotopologues as well as the spectra of the vibrational hotbands of 48TiO. In total 1034, transitions have been assigned and a line list with data of accuracy better than 10−3 cm−1 was assembled to guide future astronomical observations. Transitions from vibrationally excited states of up to have been observed in our experiments. Together with data from the literature [7], [29], [15], [47], [1], a mass-independent Dunham analysis, based on that presented in Breier et al. [7], was performed, which results in new insights into the vibrational potential of TiO.
Section snippets
Experimental approach
The experiments in this work have been performed using a laser ablation technique combined with a supersonic jet expansion, intersected with infrared radiation from a quantum cascade laser (QCL), see Witsch et al. [62] for a detailed description of the experimental setup. Here, only a brief description is given. A highly intense laser pulse of 7 ns pulse duration with an output power of up to 33.5 mJ/pulse produced by a Nd:YAG laser from Continuum Lasers (Inlite II-20 Series) was focused onto
The IR spectrum of TiO
In this study, we present the first rotationally resolved infrared spectrum of TiO between 971.2 cm−1 and 1031.9 cm−1, as shown in Fig. 1. The laser ablation source does not provide a constant molecular yield, which causes intensity fluctuations in the observed spectrum. A total of 1034 transitions were assigned to the fundamental bands of TiO and its stable titanium isotopologues, 46-50TiO, as well as vibrational hotbands of the main isotopologue 48TiO. The typical linewidth is cm−1
Isotopically invariant fitting procedure
The energy levels of the diatomic molecules, titanium monoxide and its isotopologues, can be described by the Dunham formalism [12], [13] to obtain a mass-independent molecular parameterization, according to Eq. (1). Molecular parameters of TiO were derived from a global data set analysis using high-resolution data from Breier et al. [7] and the here presented mid-IR measurements on multi-isotopologue ro-vibrational TiO transitions. The mass-independent molecular parameterization is described
Mass-independant Dunham-like parameterization
The addition of new mid-IR ro-vibrational transitions confirm and improve the former mass-independent Dunham-like parameterization [7], see Table 2. This can be seen, when comparing the equilibrium bond length of 48Ti16O () with previous works, see Table 3. The here derived value of =1.62033700(14) Å is in perfect agreement with the single isotopologue study of Ram et al. [47] (=1.62033709(25) Å) and with the former multi-isotopologue study of Breier et al. [7] (=
Conclusion
In this work, we report on 1034 ro-vibrational transitions of TiO around 1000 cm−1. Accurate experimental frequency positions with an uncertainty of better than cm−1 are provided in the supplementary material, see Tables 5–12. We have identified the fundamental bands of the stable isotopologues 46-50TiO, as well as the three lowest lying vibrational hotbands of the main isotopologue 48TiO. Our data can be used to guide the astronomical search for TiO at infrared frequencies. The tentative
CRediT authorship contribution statement
Daniel Witsch: Methodology, Conceptualization, Writing - original draft, Writing - review & editing, Visualization, Data curation, Validation, Formal analysis, Software, Investigation. Alexander A. Breier: Formal analysis, Conceptualization, Investigation, Data curation, Methodology. Eileen Döring: Data curation, Software, Investigation. Koichi M.T. Yamada: Formal analysis, Validation, Supervision. Thomas F. Giesen: Supervision, Project administration, Conceptualization, Resources, Funding
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This paper is dedicated to the 60th birthday of Stephan Schlemmer from the Universität zu Köln. His friendship and inspiring work in the field of molecular spectroscopy benefited us all. The authors gratefully acknowledge many fruitful discussions, support and advices from Stephan on various occasions. This work is supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) project number 328961117 - SFB 1319 ELCH and project number 326572190 - FU 715/2-1.
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