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Similar changes in the spectrum occur upon heating in the ν2 spectral range.
Upon increase of temperature the lines of single molecules disappear. Simultaneously, a
broad feature in the range of 1600 - 1700 cm-1 and a narrower band at 1593.4 cm-1 are
observed. The total integral intensity of spectrum (b) is only a factor of 1.7 larger as
compared with that in trace (a). This is in agreement with a small change of the infrared
intensity of the bending ν2 band of water upon clustering.26 A summary of the observed
single molecule peak positions in solid argon and assignments as well as comparison to
gas phase18 and previous work in argon12 are provided in Table 7.1.
Table 7.1: Frequencies (cm-1) and assignments of absorption lines of single H2O
molecules in the ν1, ν2 and ν3 regions in solid argon at 4 K.
Transition Spin
Isomer Gas18 In Ar,
This Work
In Ar,
Ref. 12
ν1 band
101 → 110 ortho 3674.697 3653.3 3653.38
000 → 111 para 3693.293 3669.7 3669.85
ν2 band
101 → 110 ortho 1616.711 1607.7 1607.9
000 → 111 para 1634.967 1623.6 1623.8
101 → 212 ortho 1653.267 1636.7 1636.5
ν3 band
101 → 000 ortho 3732.134 3710.8 3711.2
000 → 101 para 3779.493 3756.1 3756.49
101 → 202 ortho 3801.419 3775.9 3776.30
Previous works report fast decrease of the conversion time of the spin isomers of
water molecules upon increase of temperature of the matrix.8 Indeed, we have estimated
the equilibration time of the nuclear spin isomers at 30 K to be about 5 min. Accurate
measurements of the conversion time at higher matrix temperature are complicated by
165
Object Description
| Title | Infrared and Raman spectrosopy of molecules and molecular aggregates in helium droplets |
| Author | Sliter, Russell Thomas |
| Author email | sliter@usc.edu; sliterr@gmail.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Chemistry |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2011-04-21 |
| Date submitted | 2011 |
| Restricted until | Unrestricted |
| Date published | 2011-04-26 |
| Advisor (committee chair) | Vilesov, Andrey F. |
| Advisor (committee member) |
Reisler, Hannah Kresin, Vitaly V. |
| Abstract | This dissertation covers several different aspects of spectroscopy of molecules and molecular clusters embedded in low-temperature matrices, such as helium droplets. First, details on the formation and optimization of He droplets will be discussed. A new method of measuring droplet sizes for cw nozzle expansions using mass spectrometry was developed. The results of the measurements of the sizes of the droplets in pulsed expansion as a function of temperature will be described. Details on the electron-impact ionization of He droplets will also be discussed as well as a simple method of modeling the ionization and excitation of He atoms in the droplet. In addition, preliminary measurements on the size distribution of He droplets produced at very low temperature of 5 – 7 K in continuous expansion will be addressed.; Using matrix isolation in He droplets, vibrational spectra of clusters consisting of para-H₂ or para-H₂/D₂ have been obtained using coherent anti-Stokes Raman spectroscopy (CARS). The vibrational frequency of para-H₂ molecules obtained upon expansion of neat para-H₂/D₂ gas or liquid was found to be very similar to that in bulk solid samples having equal composition. On the other hand, spectra in clusters obtained upon expansion of 1% para-H₂/D₂ clusters seeded in He are liquid and have a considerable frequency shift, which indicate phase separation of the two isotopes in clusters at low temperature. The onset of phase separation in para-H₂/D₂ mixtures is predicted at approximately 3 K providing further evidence of super-cooled liquid hydrogen clusters.; To address the Raman spectra observed in liquid H2 clusters, vibrational and rotational spectra of bulk liquid para-H2 at temperature of T = 14 – 26 K and of solid at T = 6 – 13 K have been obtained using coherent anti-Stokes Raman scattering technique. The vibrational frequency in the liquid increases with temperature by about 2 cm⁻¹, and the shift scales with the square of the sample’s density. An extrapolation of the vibrational frequency in the metastable para-H₂ liquid below the freezing point is discussed. The results indicate that the vibron hopping between the molecules is active in the liquid, similar to that previously found in the solid.; Matrix isolation has also been performed in argon solid matrices based on a custom-made cryogenic optical cell. Single water molecules have been isolated in solid Ar matrices at 4 K and studied by ro-vibrational spectroscopy using FTIR in the regions of the v₁, v₂, and v₃ modes. Upon nuclear spin conversion at 4 K, essentially pure para-H₂O was prepared followed by subsequent fast annealing generating ice particles. FTIR studies of the vapor above the condensed water upon annealing to T ≥ 250 K indicate fast re-conversion of nuclear spin to equilibrium conditions. Our results indicate that nuclear spin conversion is fast in water dimers and larger clusters, which preclude preparation of concentrated samples of para-H₂O, such as in ice or vapor. |
| Keyword | Helium droplets; laser spectroscopy; matrix isolation; superfluidity; clusters |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m3778 |
| Contributing entity | University of Southern California |
| Rights | Sliter, Russell Thomas |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-Sliter-4404 |
| Archival file | uscthesesreloadpub_Volume23/etd-Sliter-4404.pdf |
Description
| Title | Page 189 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | Similar changes in the spectrum occur upon heating in the ν2 spectral range. Upon increase of temperature the lines of single molecules disappear. Simultaneously, a broad feature in the range of 1600 - 1700 cm-1 and a narrower band at 1593.4 cm-1 are observed. The total integral intensity of spectrum (b) is only a factor of 1.7 larger as compared with that in trace (a). This is in agreement with a small change of the infrared intensity of the bending ν2 band of water upon clustering.26 A summary of the observed single molecule peak positions in solid argon and assignments as well as comparison to gas phase18 and previous work in argon12 are provided in Table 7.1. Table 7.1: Frequencies (cm-1) and assignments of absorption lines of single H2O molecules in the ν1, ν2 and ν3 regions in solid argon at 4 K. Transition Spin Isomer Gas18 In Ar, This Work In Ar, Ref. 12 ν1 band 101 → 110 ortho 3674.697 3653.3 3653.38 000 → 111 para 3693.293 3669.7 3669.85 ν2 band 101 → 110 ortho 1616.711 1607.7 1607.9 000 → 111 para 1634.967 1623.6 1623.8 101 → 212 ortho 1653.267 1636.7 1636.5 ν3 band 101 → 000 ortho 3732.134 3710.8 3711.2 000 → 101 para 3779.493 3756.1 3756.49 101 → 202 ortho 3801.419 3775.9 3776.30 Previous works report fast decrease of the conversion time of the spin isomers of water molecules upon increase of temperature of the matrix.8 Indeed, we have estimated the equilibration time of the nuclear spin isomers at 30 K to be about 5 min. Accurate measurements of the conversion time at higher matrix temperature are complicated by 165 |
