Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S−Mo−S layers of the samples was independently determined by contact-mode atomic force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.

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Anomalous lattice vibrations of single- and few-layer MoS2.

We report ab initio calculations of the phonon dispersion relations of the single-layer and bulk dichalcogenides MoS2 and WS2. We explore in detail the behavior of the Raman-active modes A1g and E 1 2g as a function of the number of layers. In agreement with recent Raman spectroscopy measurements [C. Lee et al., ACS Nano 4, 2695 (2010)], we find that the A1g mode increases in frequency with an increasing number of layers while the E 1g mode decreases. We explain this decrease by an enhancement of the dielectric screening of the long-range Coulomb interaction between the effective charges with a growing number of layers. This decrease in the long-range part overcompensates for the increase of the short-range interaction due to the weak interlayer interaction.

/pdf/phonons-in-single-layer-and-few-layer-mos-2-and-ws-2-17zd5vlx0b.pdf

Phonons in single-layer and few-layer MoS 2 and WS 2

Phonons in single-layer and few-layer MoS2

Main-Group Element, Organic, and Organometallic Derivatives of Polyoxometalates.

Chemistry of nitric oxide relevant to biology.

We report the formation of $\mathrm{H}_{3}{}^{+}$ by proton coagulation in methanol under the impact of low energy ${\mathrm{Ar}}^{8+}$ projectiles. Our time-of-flight coincidence measurements with ${\mathrm{CH}}_{3}\mathrm{OD}$ establish that the $\mathrm{H}_{3}{}^{+}$ formation arises from intramolecular bond rearrangement of the methyl group. We have performed ab initio quantum chemical calculations that show the preferred pathway for $\mathrm{C}\mathrm{\text{\ensuremath{-}}}{\mathrm{H}}_{3}$ bond cleavage. Fragmentation of organic molecules like methanol under impact of highly charged ions is suggested as an alternative mechanism of $\mathrm{H}_{3}{}^{+}$ formation in outer space.

Formation of H3+ due to intramolecular bond rearrangement in doubly charged methanol.

The atom-atom interaction model has been extended to the case of $2H\ensuremath{-}\mathrm{Mo}{\mathrm{S}}_{2}$ and it is shown that the observed vibrational frequencies can be satisfactorily reproduced by only three force constants. However, the calculation leads to a frequency reversal in the Davydov pair which is very similar to that found earlier for $\ensuremath{\epsilon}\ensuremath{-}\mathrm{GaSe}$ and $\ensuremath{\beta}\ensuremath{-}\mathrm{GaS}$. Since Mo${\mathrm{S}}_{2}$ is somewhat less ionic than the two gallium compounds, we suggest that the frequency reversal may not be due to the long-range interactions proposed earlier. We discuss the possible role of short-range multipolar interactions.

Interlayer force and Davydov splitting in 2 H − Mo S 2

We report theoretical studies of a Λ-type five-level atomic system. The density matrix equations are set up and solved numerically to obtain the probe absorption line shape of Rb D2 transitions for cold (Doppler-free) and room temperature (Doppler-broadened) atoms. Simulated spectra for Doppler-broadened systems lead to four velocity-selective dips along with an electromagnetic induced transparency (EIT) peak as observed earlier from the co-propagating pump–probe spectroscopy of Rb D2 transitions. Effects of pump power and spontaneous decay rate from the upper levels on the simulated spectra are also studied. For cold atoms a very pronounced EIT peak is observed when the pump frequency is on resonance with one allowed transition. We find that lower pump power leads to a much sharper EIT signal in this case. A simulated dispersion curve shows a rapid variation of the refractive index that may lead to a sharp reduction of the group velocity of photons.

https://iopscience.iop.org/article/10.1088/0953-4075/40/20/008/pdf

Theoretical study of electromagnetically induced transparency in a five-level atom and application to Doppler-broadened and Doppler-free Rb atoms

It is shown that the pump-probe spectroscopy of Λ-type system for the D2 transition of 87Rb exhibits additional absorption enhancement peaks due to velocity selective optical pumping along with the electromagnetically induced transparency (EIT). The separation of these peaks is consistent with the hyperfine splitting of the upper levels. For 85Rb the peaks are not clearly discernible since the hyperfine splitting is much smaller.

https://iopscience.iop.org/article/10.1088/0953-4075/38/23/013/pdf

Velocity selective optical pumping effects and electromagnetically induced transparency for D2 transitions in rubidium

It is shown that Fizeau interferometry provides an accurate optical method to measure the refractive index and wedge angle of transparent plates used as optical components in different experiments. A near IR external cavity diode laser having spectral resolution up to 10−7 has been employed to measure the refractive index of the test plates by introducing amplitude modulation technique in the detection system of our phase shifting Fizeau interferometry. Detection of spatial fringes has been performed to find out the wedge angles of the plates by using a He–Ne laser along with the CCD-image sensor.

Pradip N. Ghosh

Papers

Formation of H3+ due to intramolecular bond rearrangement in doubly charged methanol.

Interlayer force and Davydov splitting in 2 H − Mo S 2

Theoretical study of electromagnetically induced transparency in a five-level atom and application to Doppler-broadened and Doppler-free Rb atoms

Velocity selective optical pumping effects and electromagnetically induced transparency for D2 transitions in rubidium

Direct measurement on transparent plates by using Fizeau interferometry