Chain motion and viscoelasticity in highly entangled solutions of semiflexible rods

Brownian dynamics simulations are used to study highly entangled solutions of semiflexible polymers. Bending fluctuations of semiflexible rods are signficantly affected by entanglement only above a concentration $c^{**}$, where $c^{**}\sim 10^{3}L^{-3}$ for chains of similar length $L$ and persistence length. For $c > c^{**}$, the tube radius $R_{e}$ approaches a dependence $R_{e} \propto c^{-3/5}$, and the linear viscoelastic response develops an elastic contribution that is absent for $c < c^{**}$. Experiments on isotropic solutions of $F$-actin span concentrations near $c^{**}$ for which the predicted asymptotic scaling of the plateau modulus $G \propto c^{7/5}$ is not yet valid.Comment: 4 pages, 5 figures, submitted to PR

Two-photon ionization spectroscopy and all-electron ab initio study of LiCa

Journal ArticleResonant two-photon ionization spectra of LiCa have been obtained from a laser vaporization, supersonic expansion source. The ground state of the molecule is confirmed to be X 2?+. Three band systems have been observed near 15 282, 19 310, and 22 250 cm-1 and the upper states have been assigned as 2?+, 2IIr , and, tentatively, 2II. Bond lengths and vibrational frequencies are reported for 7Li40Ca for the various states as X 2?+: r0=3.3796(11) ?, D?1/2=195.2 cm21; 2?+: r0=3.4275(47) ?, D?1/2=283.5 cm-1; 2IIr : re=3.5451(36) ?, ve5144.5 cm-1; and 2II: ve 5178.53(5) cm21. The ionization energy was also measured to be 4.471(1) eV by observing the onset of one-photon ionization. Results of ab initio all-electron calculations on twelve low-lying states [2?+(1), 2?+(2), 2?+(3), 2?+(4), 4?-(1), 4?-(1), 2II(1), 2II(2), 2II(3), 2II(4), 4II(1), and 4II(2)] are also reported, along with results on the X 1?+ ground electronic state of LiCa1. The dissociation energies of LiCa(X 2?+) into Li(2S)1Ca(1S) and of LiCa1 (X 1?+) into Li1(1S)1Ca(1S) are calculated to be 0.24 and 1.20 eV respectively, and the vertical and adiabatic ionization energies of LiCa have been determined to be almost the same, 4.43 eV, at the quadratic configuration interaction, including singles and doubles with approximate triples and all correlated electrons [QCISD(T,FULL)] level of theory

Interaction of an aluminum atom with an alkaline earth atom: spectroscopic and ab initio investigations of AICa

Journal ArticleA spectroscopic analysis of diatomic AlCa generated by laser vaporization of a 2:1 AI:Ca metal alloy followed by supersonic expansion has been completed using resonant two-photon ionization spectroscopy. Four excited electronic states have been identified and investigated in the energy region from 13 500 to 17 900 cm-1. These are the [13.5] ?II,, the [15.8] ??, the [17.0] ??3/2(?), and the [ 17.61 2?3/2 states. From rotational analysis excited state bond lengths have been measured for three of the four excited states, and the ground state has been unambiguously determined as a ?II, state with a weighted least squares value of the ground state bond length of r"0 = 3.1479 ? 0.00 10 A. The ionization energy of the molecule has also been directly determined as 5.072?0.028 eV. Ab inirio calculations for the potential energy curves of seven low-lying states of AlCa [X ?IIr, ??+, 4?-, 4IIr, ?II,(2), ??, and X l?+ and for the X 1?+ ground electronic state of AlCa+ have been carried out. In agreement with experiment, ?II, is calculated to be the ground electronic state of the neutral molecule. The dissociation energies of AlCa (X ?II,) into Al(3s?3p?,?Po) +Ca(4s?,?S) and for AlCa+ (X ??+) into A1+(3s?,?S) +Ca(4s?,?S) are calculated to be 0.47 and 1.50 eV, respectively. The excited ??+, 4?-, 4II,, 2II(2), ??, and ??+ states are calculated to lie 0.2, 0.7, 0.7, 1.1, 1.1, and 1.1 eV above X ?IIr,, respectively, and the vertical and adiabatic ionization energies of AlCa have been calculated to be 5.03 and 4.97 eV, respectively

Spectroscopic analysis of jet-cooled AICu

Journal ArticleDiatomic AlCu has been interrogated using resonant two-photon ionization spectroscopy in a supersonic expansion of helium. The ground state is shown to be X l?+, deriving from the 3&3dgd configuration, in agreement with theoretical predictions. The closed-shell nature of this molecule results in a low density of electronic states, allowing the chemical bonding and electronic structure to be investigated in detail. Five excited electronic states have been observed and characterized, leading to a potential energy diagram based solely on experimental results. Constants experimentally determined for AlCu include a ground state bond length (r0) of 2.3389?0.0004 A, a dissociation energy, Do, of 2.315?0.012 eV, and an ionization potential of 7.065?.014 eV

Spectrosopic studies of jet-cooled NiAu and PtCu

Journal ArticleSpectroscopic investigations of NiAu and PtCu have revealed that both molecules possess 2?5/2 ground electronic states, and are in this respect analogous to the isovalent molecule NiCu. The ground-state bond lengths (r?) have been measured as 2.351?0.001 ? and 2.335?0.001 ? for NiAu and PtCu, respectively. Ionization potentials have been bracketed as well, giving IP (NiAu) = 8.33 ? 0.38 eV and IP( PtCu) = 8.26 ? 0.07 eV. A reanalysis of previous high-temperature Knudsen effusion mass spectrometric data provides Doo( NiAu) = 2.52 ? 0.17 eV. The implications of these results for the electronic structure and chemical bonding of NiAu and PtCu are discussed, and comparison is made to the other diatomic metals of the nickel and copper groups

Photodissociation measurements of bond dissociation energies: Ti?+, V?+, Co?+, and Co?+

Journal ArticleThe bond dissociation energies of Ti2+, V+2 , Co+2, and Co: have been measured from the sudden onset of predissociation in the photodissociation spectra of these molecules, yielding values of D?(Ti+2)=2.435+0.002 eV, D?(V+2)=3.140?0.002 eV, D?(Co+2)=2.7651?0.001 eV, and D?(Co+3)=2.086?0.002 eV. These values are in good agreement with values previously determined from collision-induced dissociation experiments. General criteria for the interpretation of predissociation thresholds as bond dissociation energies and periodic trends in the bonding of the 3d transition metal diatomic neutrals and monocations are discussed

Optical spectroscopy of tungsten carbide (WC)

Journal ArticleResonant two-photon ionization spectroscopy has been used to study the diatomic transition-metal carbide, WC. A low-resolution scan revealed a five-member vibrational progression beginning with the 0-0 band at 17 585 cm-1. Analysis of this progression yielded a vibrational frequency of ?'e8(184W12C)=752.6(4.9) cm-1 and a bond length of r'e8(184W12C)=1.747(4) ?. Several unassigned bands were also rotationally resolved and analyzed. All of the observed bands are ?'=2??"=1 transitions, confirming the predicted ground state of 3?1 arising from a 14??8rr?15??4?116?1 configuration. The measured line positions in these bands were simultaneously fitted to provide B"=0.509 66(10) cm-1 for 184W12C, corresponding to r"0(184W12C)=1.713 5(2) ?. These values are corrected for spin-uncoupling effects in the ground state and represent our best estimate of the true bond length of WC. Dispersed fluorescence studies provide the ground-state vibrational constants of ?e=983(4) cm-1 and ?exe=11(1) cm-1, and have also permitted the low-lying [1.2] 3?2 and [4.75] states to be located and characterized. These results on WC are discussed in relation to the isovalent molecule MoC and other transition-metal carbides

Spectroscopic studies of jet-cooled AINi

Journal ArticleResonant two-photon ionization spectroscopy has been used to interrogate diatomic AlNi produced by laser vaporization of a 1:l alloy target in a supersonic molecular beam of helium. Although a large density of states in this molecule prohibits a concise elucidation of its electronic structure, the presence of discrete transitions has allowed several bands to be rotationally resolved. From the analysis of these bands the ground state has been determined as X 2?5/2, originating from the 3SA3&Nio2 configuration, and the bond length has been measured as 2.3211?0.0007 A. The dissociation energy and ionization potential of AlNi have also been determined as D0( AlNi) =2.29?0.05 eV and I.P. ( AlNi) =6.95?0.09 eV, respectively