Effects Of Permanent Dipole Moments Research Articles

Permanent dipole moment induced stability and group index switching in a three-level molecule

The effect of permanent dipole moments on the optical bistability of weak probe light in a unidirectional ring cavity with a three-level molecule is investigated. It is found that the intensity threshold of optical bistability and optical multistability can be controlled by quantum coherence induced by permanent dipole moments. It is shown that switching from optical bistability to optical multistability and lasing without population inversion can be occurred by this coherence under different conditions.

Dissociation dynamics ofHD+in intense femtosecond laser pulses by one-dimensional wave-packet propagation

Dissociation of ${\text{HD}}^{+}$ molecular ion by intense femtosecond laser pulses is studied theoretically using quantum propagation of nuclear vibrational wave packets. Radiative pulses of peak intensity in the range of ${10}^{14}--{10}^{15}\text{ }\text{W}/{\text{cm}}^{2}$ with carrier wavelength of 800 nm are applied on the system. In our calculation we have used a four component wave packet in two spatial regions for studying the dynamics of dissociated fragments and the nondissociated molecules simultaneously. This technique enables us to investigate the dynamics by varying the pulse duration (15--75 fs) and the peak amplitude of the field for the initial vibrational state ${\ensuremath{\nu}}_{i}=0$. The effect of permanent dipole moments and nonadiabatic mixing coefficients of the system on dissociation probability and the branching ratio of photofragments between two outgoing channels are also discussed. Studies of kinetic energy spectra help us to estimate the number of photon absorption and shift of the field induced resonances from the field free levels. The dissociation spectrum at high intensity has been compared with available experimental results.

Effects of permanent dipole moments on stimulated Raman adiabatic passage

The effects of permanent dipole moments on stimulated Raman adiabatic passage (STIRAP) are considered. Analytic expressions for the Hamiltonian including the effects of permanent dipole moments are developed for the STIRAP process. The potential detrimental effect of permanent dipoles on population transfer using standard STIRAP techniques is demonstrated using model three-level systems. However, the presence of permanent dipole moments can allow the utilization of alternative multi-photon mechanisms for STIRAP. Here two-photon plus two-photon STIRAP is highlighted as a potential new mechanism.

Effects of permanent dipole moments in transient four-wave mixing experiments

A two-pulse degenerate four-wave mixing experiment is analyzed in the case where the medium under investigation can be modeled by two-level systems having unequal permanent dipole moments. By modeling the light pulses by double exponentials [exp(-Gamma/t/)], we give an analytical expression of the third-order nonlinear polarization of the medium. We apply this result to simulate the measured signal in such experiment. We show that in the case of a two-photon transition, a signal can be detected if the pump pulse interacts with the medium before the probe pulse contrary to what is observed for excitations in the resonance region. An attempt to explain this behavior is made and the detected signal is analyzed in terms of pure coherent processes. This effect appears as a signature of the presence of permanent dipole moments. To test this property on a more realistic system, we then have considered a one-dimensional frequency-selected infrared degenerate four-wave mixing experiment on a molecular anharmonic vibrational mode modeled by a Morse potential and coupled to a dissipative bath of harmonic oscillators. We show that the two-photon transitions allowed by the presence of permanent dipole moments enable to analyze the multilevel system dynamics as if they were the one of a two-level system. Our results can also be extended to the case of inhomogeneous broadening and are of interest to study the infrared photon-echo response of anharmonic vibrational modes.

Coherent population accumulations of multiphoton transitions induced by an ultrashort pulse train in polar molecules

Coherent population accumulations of multiphoton transitions induced by an ultrashort pulse train in a two-level polar molecule are investigated theoretically by solving the density-matrix equations without invoking any of the standard approximations. It is shown due to the effects of permanent dipole moments, that the population accumulation of multiphoton transitions can be obtained in the polar molecule. Moreover, the population accumulations depend crucially on the relative phase between two sequential pulses, and the period in which the maximum population accumulation occurs is $2\ensuremath{\pi}∕N$ in $N$-photon transitions.

Enhancement of ultrafast four-wave mixing in a polar molecule medium

We investigate the ultrafast four-wave mixing (FWM) with two-color few-cycle ultrashort pulses propagating in a two-level polar molecule medium. It is found that the enhancement of FWM can be achieved even for low intensity pulses due to the effects of permanent dipole moments (PDM) in polar molecules. Moreover, the conversion efficiency of FWM can be controlled by the carrier-envelope phases (CEP) of two ultrashort pulses.

Dipolar effects on propagation of ultrashort laser pulse in one-dimensional para-nitroaniline (pNA) molecules

The dynamic behaviour of ultrashort (femtosecond) laser pulses in a molecular medium is studied by solving the full Maxwell–Bloch equations beyond the limits of the slowly varying envelope approximation and the rotating-wave approximation under the resonant and the non-resonant conditions. A one-dimensional asymmetric charge-transfer molecule, para-nitroaniline, is used as a model molecule whose electronic properties are calculated with the time-dependent hybrid density functional theory. Under the one-photon resonant condition, 4π pulse is separated into two sub-pulses. The weight of the second-harmonic component mainly contributed by the two-photon excitation becomes stronger with longer propagation time. Under the two-photon resonant condition, the separation of 4π pulse is not induced and many higher-order spectral components beyond the second-harmonic generation occur. Interestingly, when the pulse propagates for long enough, the carrier modification becomes so significant that a continuous spectrum is generated. The Fourier transform of the high-harmonic spectrum demonstrates that an even shorter laser pulse can be produced in both resonant and non-resonant propagations. The effects of permanent dipole moments on the pulse evolution are discussed.

Dynamical behavior of ultrashort pulse laser in para-nitroaniline molecules

The electronic structures and dipole moments of para- nitroaniline molecule are calculated by use of density functional theory at ab initio level. We then inve stigate theoretically the interaction between the ultrashort pulse laser and the para-nitroaniline molecule medium by solving the full Maxwell-Bloch equations. The effects of permanent dipole moments on the pulse reshaping, the spectrum com ponents, and the evolutions of energy state populations are discussed in detail. The results show that the standing slowly varying envelope and the rotating-wav e approximations fail to give a good description for the propagation of an ultra short pulse in the molecule medium. Furthermore, the dipole moments of the molec ule make the propagation deviate from the area theorem further and make the pul se split more quickly. When the pulse with the excitation energy of the charge-t ransfer state propagates in the para-nitroaniline molecule medium, the second-ha rmonic spectral component appears obviously and becomes stronger as the propagat ing distance increases, which displays the high two-photon absorption properties of the molecule.

The effect of permanent dipole moments of adsorbates upon I– V characteristics of a bilayer tunneling junction between self-assembled monolayers on an Au(1 1 1) substrate and a gold tip

It is important to understand an electronic property of an interface between an organic material and a metal electrode. In the present work, we measured current–voltage ( I– V) curves of self-assembled monolayers (SAMs) on Au(1 1 1) using a conducting atomic force microscope (AFM) with chemically modified Au-coated AFM tips. This contact resulted in a bilayer junction between the Au(1 1 1) substrate covered with one SAM and the Au-coated tip with the other SAM. An alkanethiol (octanethiol) and benzenemethanethiols with various terminal groups (–H, –CH 3, –Cl, –CF 3) were used as the adsorbates. The shapes of the I– V curves depended on the terminal groups. This phenomenon was attributed to the change in the work function of gold due to different permanent dipole moments of the terminal groups.

On the effects of absolute laser phase on the interaction of a pulsed laser with polar versus nonpolar molecules

The absolute laser phase dependence of the time-dependent populations of the molecular states, including the steady-state (long time) populations of the states, associated with the interaction of a molecule with a pulsed laser is investigated using illustrative two-level examples. One-photon transitions, including the effects of permanent dipoles, are discussed as a function of the pulse duration, intensity, and (absolute) laser phase, for selected laser frequencies. The effects of laser phase can be large, depending on the values of the pulse duration for a given frequency and intensity. The effects of permanent dipoles, relative to no permanent dipoles, are significant for large laser field strengths ε0. When the laser-molecule coupling parameter b=μ12ε0/E21⩾0.2, where μ12 and E21 are the transition dipole and energy difference between the ground and excited states, respectively, the dynamics of the pulse-molecule interaction are (strongly) phase dependent, independent of pulse duration, whereas the corresponding steady-state populations of the molecular states may or may not be phase-dependent depending on the pulse duration. Analytical rotating wave approximations for pulsed laser-molecule interactions are useful for interpreting the dynamics and the steady-state results as a function of field strength and pulse duration, including the effects of permanent dipole moments. The results reported in this paper are based on molecular parameters associated with an S0→S1 electronic transition in a dipolar molecule. However, they are presented in reduced form and therefore can be scaled to other regions of the electromagnetic spectrum. Short, intense pulses at or beyond the limits of current laser technology will often be required for the types of absolute laser phase effects of this paper to be appreciable for electronic excitations. The discussion, in the UV-VIS, also suffers from the use of a two-level model and from the requirement of field intensities that can be beyond the Keldysh limit. For other spectral regions, these absolute laser phase effects will be much more readily applicable.

Effects of permanent dipole moments on pulsed laser-induced multiphoton resonances in a two-level system

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Permanent dipole moments and two-color multiphoton resonances in the two-level molecule: The rotating wave approximation versus exact results

The interaction of two continuous wave electric fields (lasers) with a two-level molecule, that has a nonzero difference d, between the permanent dipoles of the two states involved in a multiphoton two-color transition, is studied in the rotating wave approximation and by using exact methods. Analytical rotating wave approximation results for the time-dependent populations of the molecular states and for the resonance profiles are derived and used to help discuss the effects of permanent dipole moments in the two-color absorption problem. Several examples of multiphoton, two-color, two-level resonance profiles calculated in the rotating wave approximation and by using exact techniques, are employed for illustrative purposes. They are used to help quantify the conditions for the validity of the two-color rotating wave approximation, which are considerably more restrictive than the analogous conditions for the one-color rotating wave approximation with d≠0, and to illustrate the interpretive and predictive nature of the rotating wave approximation results. For example, there are many multiphoton, two-color transitions that are available if a molecule possesses permanent dipoles (d≠0). The analytic rotating wave approximation expression for the molecule-two laser coupling can often be used to reliably estimate the field parameters of the two lasers that are required to optimize a given transition relative to others.

Effects of permanent dipole moments in degenerate four-wave-mixing processes.

The main goal of this paper is to analyze a two-photon degenerate four-wave-mixing process that occurs in an isotropic media modeled by homogeneously broadened two-level systems having unequal permanent dipole moments and subject to rotational diffusion. Our theoretical treatment is based upon a third-order perturbative solution of the density-matrix equations and does not require the rotating-wave approximation. The medium being excited by three stationary fields, we give an analytical expression of the macroscopic polarization up to the third order, including permanent dipole moments, orientational diffusion, and field polarizations. We apply these general results to the description of a degenerate four-wave-mixing process by selecting, in the expressions, the contributions in the direction ${\mathbf{k}}_{\mathrm{\ensuremath{\alpha}}}$-${\mathbf{k}}_{\mathrm{\ensuremath{\beta}}}$+${\mathbf{k}}_{\ensuremath{\gamma}}$, ${\mathbf{k}}_{\mathrm{\ensuremath{\alpha}}}$, ${\mathbf{k}}_{\mathrm{\ensuremath{\beta}}}$, and ${\mathbf{k}}_{\ensuremath{\gamma}}$ being, respectively, the wave vectors of the exciting fields. We show that the two-photon degenerate four-wave mixing is dependent on the permanent dipole moments and is not present if these dipole moments are equal or zero. We analyze the influence of rotational diffusion. Different cases of field polarizations are also considered, and we discuss a comparison of the well-known one-photon degenerate four-wave-mixing process and this two-photon process. This process is also discussed in the particular case of phase conjugation.

Effects of permanent dipole moments on continuous-wave laser-induced multiphoton resonances in a two-level system

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Effects of permanent dipole moments on the collision-free interaction of a two-level system with a laser and a static electric field.

The consequences of having state-specific diagonal transition matrix elements on the interaction of a two-level system with a laser field and a static electric field are examined within the dipole and rotating-wave approximations. A finite difference between the permanent dipole moments can result in the appearance of subsaturating ancilliary fringes flanking the usual Lorentzian transition resonance. The requisite conditions for a two-level system to exhibit an oscillatory line shape are discussed. These fringes arise from the essential transparency of the system to the field at prescribed intensity-dependent frequencies.

Theory of the spontaneous polarization of the free surface of a dielectric

The electric surface modes at the free surface of a dielectric may induce, through a non-linear mechanism, a static polarization of this surface. An estimate of this effect shows that the corresponding difference of potential may be non-negligible with respect to the usual value arising in situations where orientation effects of permanent dipole moments are present.

Effect of Permanent Dipole Moments on the Hypochromism in Helical Polymers

The theory of hypochromicity in the ordered polymer is revised by taking into account the effect of nonvanishing permanent dipole moment of each component monomer. It is found by numerical calculations on the single stranded helical poly-C and the α-helical polypeptide that the effect contributes a considerable amount to hypochromicity. The ratios of the helix absorption to base absorption are 0.75 and 0.70 respectively in the former and the latter cases, compared to those ratios 0.54 and 0.82 obtained by neglecting the present effect.