Vibrational phase relaxation along the critical isochore of nitrogen: the role of local density fluctuations in the rate enhancement

Vibrational dephasing of the nitrogen molecule is known to show highly interesting anomalies near its gas-liquid critical point. Here we present theoretical and computational studies of the Raman linewidth of nitrogen along the critical isochore. The linewidth is found to have a lambda-shaped temperature dependence near the critical point. As observed in experimental studies, the calculated line shape becomes Gaussian as the critical temperature (Tc) is approached. Both the present simulation and a mode coupling theory analysis show that the slow decay of the enhanced density fluctuations near the critical point, probed at the subpicosecond time scales by vibrational frequency modulation, along with an enhanced vibration-rotation coupling, are the main causes of the observed anomalies

Non-monotonic composition dependence of vibrational phase relaxation rate in binary mixtures

We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational phase relaxation in a model system to explore the unusual features arising due to concentration fluctuations which are absent in one component systems. The model studied consider strong attractive interaction between the dissimilar species to discourage phase separation. The model reproduces the experimentally observed nonmonotonic, nearly symmetric, composition dependence of the dephasing rate. In addition, several other experimentally observed features, such as the maximum of the frequency modulation correlation time τc at mole fraction near 0.5 and the maximum rate enhancement by a factor of about 3 above the pure component value, are also reproduced. The product of mean square frequency modulation [<Δω2(0)>] with τc indicates that the present model is in the intermediate regime of inhomogeneous broadening. The nonmonotonic composition χA dependence of the dephasing time τv is found to be primarily due to the nonmonotonic χ dependence of τc, rather than due to a similar dependence in the amplitude of <Δω2(0)>. The probability distribution of Δω shows a markedly non-Gaussian behavior at intermediate composition (χA ≈ 0.5). We have also calculated the composition dependence of the viscosity in order to explore the correlation between the composition dependence of viscosity η with that of τv and τc. It is found that both the correlation time essentially follow the composition dependence of the viscosity. A mode coupling theory is presented to include the effects of composition fluctuations in binary mixture

Simulation and theory of vibrational phase relaxation in the critical and supercritical nitrogen: Origin of observed anomalies

We present results of extensive computer simulations and theoretical analysis of vibrational phase relaxation of a nitrogen molecule along the critical isochore and also along the gas-liquid coexistence. The simulation includes all the different contributions [atom-atom (AA), vibration-rotation (VR) and resonant transfer] and their cross-correlations. Following Everitt and Skinner, we have included the vibrational coordinate ($q$) dependence of the interatomic potential. It is found that the latter makes an important contribution. The principal important results are: (a) a crossover from a Lorentzian-type to a Gaussian line shape is observed as the critical point is approached along the isochore (from above), (b) the root mean square frequency fluctuation shows nonmonotonic dependence on the temperature along critical isochore, (c) along the coexistence line and the critical isochore the temperature dependent linewidth shows a divergence-like $\lambda$-shape behavior, and (d) the value of the critical exponents along the coexistence and along the isochore are obtained by fitting. The origin of the anomalous temperature dependence of linewidth can be traced to simultaneous occurrence of several factors, (i) the enhancement of negative cross-correlations between AA and VR contributions and (ii) the large density fluctuations as the critical point (CP) is approached. The former makes the decay faster so that local density fluctuations are probed on a femtosecond time scale. A mode coupling theory (MCT) analysis shows the slow decay of the enhanced density fluctuations near critical point. The MCT analysis demonstrates that the large enhancement of VR coupling near CP arises from the non-Gaussian behavior of density fluctuation and this enters through a nonzero value of the triplet direct correlation function.Comment: 35 pages, 15 figures, revtex4 (preprint form

Vibrational phase relaxation of O-H stretch in bulk water: role of large amplitude angular jumps and negative cross-correlations among the forces on the O-H bond

The ultrafast vibrational phase relaxation of O-H stretch in bulk water is investigated in molecular dynamics simulations. The dephasing time (T2) of the O-H stretch in bulk water calculated from the frequency fluctuation time correlation function (C(t)) is in the range of 70-80 femtosecond (fs), which is comparable to the characteristic timescale obtained from the vibrational echo peak shift measurements using infrared photon echo [W.P. de Boeij, M.S. Pshenichnikov, D.A. Wiersma, Ann. Rev. Phys. Chem. 49 (1998) 99]. The ultrafast decay of Cω(t) is found to be responsible for the ultrashort T2 in bulk water. Careful analysis reveals the following two interesting reasons for the ultrafast decay of Cω(t). (A) The large amplitude angular jumps of water molecules (within 30-40 fs time duration) provide a large scale contribution to the mean square vibrational frequency fluctuation and gives rise to the rapid spectral diffusion on 100 fs time scale. (B) The projected force, due to all the atoms of the solvent molecules on the oxygen (FO(t)) and hydrogen (FH(t)) atom of the O-H bond exhibit a large negative cross-correlation (NCC). We further find that this NCC is partly responsible for a weak, non-Arrhenius temperature dependence of the dephasing rate

Vibrational phase relaxation of O–H stretch in bulk water: Role of large amplitude angular jumps and negative cross-correlations among the forces on the O–H bond

The ultrafast vibrational phase relaxation of O–H stretch in bulk water is investigated in molecular dynamics simulations. The dephasing time (T2) of the O–H stretch in bulk water calculated from the frequency fluctuation time correlation function (Cω(t)) is in the range of 70–80 femtosecond (fs), which is comparable to the characteristic timescale obtained from the vibrational echo peak shift measurements using infrared photon echo [W.P. de Boeij, M.S. Pshenichnikov, D.A. Wiersma, Ann. Rev. Phys. Chem. 49 (1998) 99]. The ultrafast decay of Cω(t) is found to be responsible for the ultrashort T2 in bulk water. Careful analysis reveals the following two interesting reasons for the ultrafast decay of Cω(t). (A) The large amplitude angular jumps of water molecules (within 30–40 fs time duration) provide a large scale contribution to the mean square vibrational frequency fluctuation and gives rise to the rapid spectral diffusion on 100 fs time scale. (B) The projected force, due to all the atoms of the solvent molecules on the oxygen (FO(t)) and hydrogen (FH(t)) atom of the O–H bond exhibit a large negative cross-correlation (NCC). We further find that this NCC is partly responsible for a weak, non-Arrhenius temperature dependence of the dephasing rate

Vibrational Phase Relaxation along the Critical Isochore of Nitrogen: The Role of Local Density Fluctuations in the Rate Enhancement

Vibrational dephasing of the nitrogen molecule is known to show highly interesting anomalies near its gas-liquid critical point. Here we present theoretical and computational studies of the Raman linewidth of nitrogen along the critical isochore. The linewidth is found to have a lambda-shaped temperature dependence near the critical point. As observed in experimental studies, the calculated line shape becomes Gaussian as the critical temperature (Tc) is approached. Both the present simulation and a mode coupling theory analysis show that the slow decay of the enhanced density fluctuations near the critical point, probed at the subpicosecond time scales by vibrational frequency modulation, along with an enhanced vibration-rotation coupling, are the main causes of the observed anomalies

Mg-Al layered double hydroxide-methotrexate nanohybrid drug delivery system: Evaluation of efficacy

Mg-Al layered double hydroxide nanoparticles were synthesized by one-pot co-precipitation method and anticancerous drug methotrexate was incorporated into it by in-situ ion exchange. The LDH-MTX nanohybrid produced moderately stable suspension in water, as predicted by zeta potential measurement. X-ray diffraction revealed that the basal spacing increased to nearly twice the same for pristine LDH on MTX intercalation. Thermogravimetric analyses confirmed an increase in thermal stability of the intercalated drug in the LDH framework. A striking enhancement in efficacy/sensitivity of MTX on the HCT-116 cells was obtained when intercalated within the LDH layers, as revealed by the attainment of half maximal inhibitory concentration of LDH-MTX nanohybrid by 48 h, whereas, bare MTX required 72 h for the same. The MTX release from MgAl-LDH-MTX hybrids in phosphate buffer saline at pH 7.4 followed a relatively slow, first order kinetics and was complete within 8 days following diffusion and crystal dissolution mechanism. (C) 2013 Elsevier B.V. All rights reserved

Phonon Localization and Entropy-Driven Point Defects Lead to Ultralow Thermal Conductivity and Enhanced Thermoelectric Performance in (SnTe)1–2x(SnSe)x(SnS)x\mathrm{(SnTe)_{1–2x}(SnSe)_{x}(SnS)_{x}}

Understanding of phase stability, chemical bonding, and phonon transport are essential to realize ultralow thermal conductivity in crystalline solids for designing high-performance thermoelectric (TE) materials. Pristine SnTe, a homologue of PbTe, exhibits poor TE performance primarily because of high lattice thermal conductivity, κlat. Herein, the amorphous limit of $κ_{lat}$ is achieved via engineering configurational and vibrational entropies in pseudoternary $\mathrm{(SnTe)_{1–2x}(SnSe)_{x}(SnS)_{x}}$. Density functional theory calculations and synchrotron X-ray pair distribution function analysis reveal that S atoms are locally off-centered in global cubic SnTe, resulting in a low-energy localized optical phonon which strongly couples with heat-carrying acoustic phonons. Additionally, substitution of Se and S in SnTe increases the configurational entropy and point defects, resulting in an ultralow $κ_{lat}$ of 0.52 W/mK. Finally, improvement of the Seebeck coefficient is achieved via the synergistic effect of resonant doping (In) and valence band convergence (Ag), which lead to a high TE figure of merit, zT, of ∼1.3 at 854 K

Histologic pattern, bilaterality and clinical evaluation of 957 ovarian neoplasms: A 10-year study in a tertiary hospital of eastern India

Objective: The aim was to study the distribution of morphological pattern of benign and malignant ovarian neoplasms in different age groups in eastern India and to determine the likelihood of bilateral involvement in different morphologic subtypes. Materials and Methods: 957 cases of ovarian tumors were studied over a period of 10 years (from January 2001 to December 2010). Results: Most of the benign tumors occurred between 20 and 40 years of age, while the malignant lesions presented commonly between 41 and 50 years. The most common histological types were serous cystadenoma (29.9%), followed by mature teratoma (15.9%) and mucinous cystadenoma (11.1%). Major proportion of malignant ovarian tumors was contributed by surface epithelial tumors (60.9%). Serous cystadenocarcinoma was the predominant malignant tumor (11.3%). Metastatic tumors were found to involve the bilateral ovaries in 72%, while 49.5% of malignant serous tumors were bilateral. Borderline serous tumors showed bilateral involvement more commonly (27.4%) than borderline mucinous tumors (15.7%). Most of the malignant tumors presented as stage III (60%) or stage II (20%) disease. The overall survival rate was 85% for stage I tumors, 65% for stage II, 30% for stage III and 15.5% for stage IV tumors. Conclusion: We noted an earlier age at presentation of malignant tumors. Mature teratoma was found to be the second most common benign tumor (after serous cystadenoma). We also noted a lower percentage of endometrioid tumors. Lower number of stage IV tumors was noted, with a significant number of malignant ovarian tumors presenting at an earlier age