Two-photon Pathways Research Articles

Two-photon pathway to ultracold ground state molecules of 23Na40K

We report on high-resolution spectroscopy of ultracold fermionic 23Na40K Feshbach molecules, and identify a two-photon pathway to the rovibrational singlet ground state via a resonantly mixed B1Π ∼ c3Σ+intermediate state. Photoassociation in a 23Na–40K atomic mixture and one-photon spectroscopy on 23Na40K Feshbach molecules reveal about 20 vibrational levels of the electronically excited c3Σ+state. Two of these levels are found to be strongly perturbed by nearby B1Π levels via spin–orbit coupling, resulting in additional lines of dominant singlet character in the perturbed complex , or of resonantly mixed character in . The dominantly singlet level is used to locate the absolute rovibrational singlet ground state via Autler–Townes spectroscopy. We demonstrate coherent two-photon coupling via dark state spectroscopy between the predominantly triplet Feshbach molecular state and the singlet ground state. Its binding energy is measured to be 5212.0447(1) cm−1, a thousand-fold improvement in accuracy compared to previous determinations. In their absolute singlet ground state, 23Na40K molecules are chemically stable under binary collisions and possess a large electric dipole moment of 2.72 Debye. Our work thus paves the way towards the creation of strongly dipolar Fermi gases of NaK molecules.

Pathway competition of H+ in intense femtosecond laser fields

We experimentally measured the kinetic energy and angular distributions of fragment ion H+ of H2 as a function of 810 nm femtosecond laser intensity by using velocity map imaging technique. The reasonable origination of dissociation channels (1.0) and (1.1) are proposed. The analysis of the angular distribution indicates the net two-photon pathway via the 3ω crossing dominates over the direct one-photon pathway in channel (1.0). The relative yield of fragment peaks indicates that dissociation and ionization of H2+ are competitive. The lower laser intensities emphasize the dissociation probability of H2+, and the higher laser intensities favor higher ionization stages.

Pathway of H+ in fragmentation of H2 in 810 nm intense femtosecond laser fields

We experimentally measured the kinetic energy and angular distributions of fragment ion H+ of H2 as a function of 810 nm femtosecond laser intensity by using velocity map imaging technique. The reaction pathways of dissociation channels H2+ → H+ + H and H22+ → H+ + H+ are proposed. The analysis of the angular distribution indicates the net two-photon pathway via the 3ω crossing dominates over the direct one-photon pathway in channel H2+ → H+ + H. The ionization of H2+ to H22+ may be initiated by bond softening and above-threshold dissociation. The relative yield of fragment peaks indicates that dissociation and ionization of H2+ are competitive. The lower laser intensities emphasize the dissociation probability of H2+, and the higher laser intensities favor higher ionization stages.

Quantum interference control of a four-level diamond-configuration quantum system

We investigate coherent control of the two-photon transition pathways of a four-level atomic system in a diamond configuration. When an ultrashort laser pulse interacts with this system in the ground state 5S1/2 of rubidium, the two-photon transition probability amplitude of 5D3/2 is obtained by a summation of all possible resonant and nonresonant two-photon transition probability amplitudes via 5P1/2 and 5P3/2. Second-order perturbation theory predicts that the maximal constructive interference of the transition probability amplitudes occurs when the phases of eight different spectrum blocks satisfy four different phase relations. Experiments carried out with spectrally phase-coded laser pulses show good agreement with the theoretical prediction.

Optical bistability and multistability in a parametric region

We study the optical bistability (OB) and optical multistability (OM) behaviors in a five-level $$\Lambda $$ -type parametric region atomic system by two-photon resonant transitions. We find that the intensity threshold of OB and switching from OB to OM or vice versa can be controlled via quantum interference between different two-photon transitions pathways.

UV/Vis and NIR Light-Responsive Spiropyran Self-Assembled Monolayers

Self-assembled monolayers of a 6-nitro BIPS spiropyran (SP) modified with a disulfide-terminated aliphatic chain were prepared on polycrystalline gold surfaces and characterized by UV/vis absorption, surface-enhanced Raman scattering (SERS), and X-ray photoelectron spectroscopies (XPS). The SAMs obtained are composed of the ring-closed form (i.e., spiropyran) only. Irradiation with UV light results in conversion of the monolayer to the merocyanine form (MC), manifested in the appearance of an N(+) contribution in the N 1s region of the XPS spectrum of the SAMs, the characteristic absorption band of the MC form in the visible region at 555 nm, and the C-O stretching band in the SERS spectrum. Recovery of the initial state of the monolayer was observed both thermally and after irradiation with visible light. Several switching cycles were performed and monitored by SERS spectroscopy, demonstrating the stability of the SAMs during repeated switching between SP and MC states. A key finding in the present study is that ring-opening of the surface-immobilized spiropyrans can be induced by irradiation with continuous wave NIR (785 nm) light as well as by irradiation with UV light. We demonstrate that ring-opening by irradiation at 785 nm proceeds by a two-photon absorption pathway both in the SAMs and in the solid state. Hence, spiropyran SAMs on gold can undergo reversible photochemical switching from the SP to the MC form with both UV and NIR and the reverse reaction induced by irradiation with visible light or heating. Furthermore, the observation of NIR-induced switching with a continuous wave source holds important consequences in the study of photochromic switches on surfaces using SERS and emphasizes the importance of the use of multiple complementary techniques in characterizing photoresponsive SAMs.

Temporal coherent control of superfluorescent pulses

We demonstrate that collective atomic interferences can be investigated by measuring the superfluorescence (SF) time delay. A pair of broadband (≈20 nm), ultrashort (≈80 fs), collinear pulses with a variable delay coherently excites rubidium (Rb) atoms. The generated superfluorescent pulses at 420 nm on the cascade transition are recorded by a picosecond streak camera. Both intensity and SF time delay of the 420 nm pulse are altered as the delay between input pulses varies. In particular, the SF time delay of the normalized 420 nm pulse exhibits oscillations with different periods. This can be understood in terms of atomic and quantum interferences due to two possible two-photon excitation pathways through the intermediate levels (Rb D-lines).

Coherent photocurrent control in a magnetic field through quantum interference

We treat theoretically the coherent optical control of carrier and current densities in a semiconductor quantum well in the presence of a magnetic field perpendicular to the plane of the well by means of time-dependent perturbation theory. Photocurrents of electrons and holes are shown to be generated through quantum interference of one- and two-photon excitation pathways when the sample is exposed to two-color pulses, and we find that these currents rotate in time in opposite directions as a result of excitation by these pathways to or from different but adjacent Landau levels. The initial directions of the currents can be controlled by adjusting a relative phase parameter of the optical pulses. The magnitudes of the generated photocurrents are comparable to those predicted and detected in the absence of a magnetic field, and so the effects considered here should be observable.

Electromagnetically induced transparency in a five-levelΛsystem dominated by two-photon resonant transitions

We study the steady optical response of a five-level atomic system in the parametric region where resonant two-photon transitions are much stronger than far-detuned single-photon transitions. We find that the concurrent absorption of two weak probe fields can be well suppressed in a narrow spectral region to attain electromagnetically induced transparency (EIT) via quantum destructive interference between different two-photon transition pathways. To gain a deeper insight into relevant physics, we adiabatically reduce this five-level system with trivial single-photon transitions into a three-level system with vanishing single-photon transitions by deriving an effective Hamiltonian. The two systems have almost the same two-photon absorption spectra exhibiting typical EIT features but are a little different in fine details. This means that most characteristics of two-photon quantum destructive interference are reserved after the adiabatic elimination approximation. In addition, we verify by numerical calculations that the two-photon EIT spectra are insensitive to the dipole-dipole interaction of cold Rydberg atoms when the uppermost level has a high principle quantum number.

Stimulated coherent anti-Stokes Raman spectroscopy (CARS) resonances originate from double-slit interference of two-photon Stokes pathways.

Coherent anti-Stokes Raman spectroscopy (CARS) uses vibrational resonances to study nuclear wavepacket motions and is widely used in cell imaging and other applications. The resonances usually lie on top of a parametric component that involves no change in the molecular state and creates an undesirable background which reduces the sensitivity of the technique. Here, by examining the process from the perspective of the molecule, rather than the field, we are able to separate the two components and recast each resonance as a modulus square of a transition amplitude which contains an interference between two Stokes pathways, each involving a different pair of field modes. We further propose that dissipative signals obtained by measuring the total absorption of all field modes in a convenient collinear pulse geometry can eliminate the parametric component and retain the purely resonant contributions. Specific vibrational resonances may then be readily detected using pulse shapers through derivatives with respect to pulse parameters.

Photoionization of DNA and RNA Bases, Nucleosides and Nucleotides Through a Combination of One- and Two-photon Pathways upon 266 nm Nanosecond Laser Excitation¶

The 266 nm nanosecond laser photolysis of various purine and pyrimidine derivatives results in their photoionization (PI) as one of the primary photochemical pathways. Electron photoejection occurs through a combination of one- and two-photon mechanisms. The PI values depend on the substituents attached to the chromophore of the base. The net PI of the purine bases at 266 nm are of the same order of magnitude (10−2) as those of the pyrimidine bases under similar experimental conditions. The monophotonic component is approximately one-third of the net PI yield of the bases. A nonrelaxed singlet excited state intermediate is tentatively proposed for this pathway. It is proposed that this state is significantly stabilized by water solvation, transforming it into a charge transfer to solvent state from which the hydrated electron evolves.

Enhanced selectivity and yield in multichannel photodissociation reactions: Application to CH3I

We develop a method to improve the population transfer and final-channel control of multichannel photodissociation reactions. The method is applied to the photodissociation of methyl iodide, CH3(v)+I*(2P1/2)<--CH3I-->CH3(v)+I(2P3/2). Our method is based on simultaneously exciting many two-photon pathways that lead to the same final outcome, each proceeding via a different intermediate bound state. The selectivity of the final product state(s) is a result of coherently controlled interference between the quantum pathways. The improvement in the population transfer yield from the ground state to the selected dissociative channel(s) is made possible by executing the process in an adiabatic fashion.

Role of Sequence and Conformation on the Photochemistry and the Photophysics of A−T DNA Dimers: An Experimental and Computational Approach

The role of base sequence and conformation on the photochemistry and photophysics of thymidylyl (3'-5')-2'-deoxyadenosine sodium salt (TpdA) and 2-deoxyadenylyl (3'-5')-thymidine ammonium salt (dApT) was studied. To this end, nanosecond transient absorption at 266 nm, steady-state irradiation at 254 nm, and quantum chemical calculations were used. The transient absorption spectra show the solvated electron broad band in the visible region for each dinucleotide. In addition, low-intensity absorption bands are observed in the UV region, which are attributed to the deprotonated and protonated neutral radicals of adenine and thymine bases. Photoionization (PI) occurs by one- and two-photon pathways; the latter accounting for approximately 70% of the net PI yield. A diffusion-limited rate constant of 2.0 x 10(10) M(-1) s(-1) was obtained for the reaction of the neutral molecule with the photoejected electron in both sequences. The photodestruction yield, measured from the chromophore loss at 260 nm, decreases in the presence of well-known electron scavengers. This suggests the participation of base radical anions as one of the photodegradation pathways, which is higher in TpdA than in dApT. The intermediacy of a radical ion pair (charge separated state) between the adjacent adenine and thymine bases is proposed in the formation of the [2 + 2] cycloadduct intermediate. The [2 + 2] cycloadduct intermediate is known to be the precursor of the thymine-adenine eight-member ring photoproduct (TA*). Conformational constrains in the radical ion pair are suggested to explain the absence of the TA* photoproduct in dApT. This hypothesis is supported by semiempirical calculations performed on all relevant reactive intermediates proposed to participate in the mechanism of formation of TA*. Altogether, the results show that sequence and conformation profoundly influence the photochemistry and the photophysics of these DNA model systems.

Excitonic effects on the two-color coherent control of interband transitions in bulk semiconductors

Quantum interference between one- and two-photon absorption pathways allows coherent control of interband transitions in unbiased bulk semiconductors; carrier population, carrier spin polarization, photocurrent injection, and spin current injection may all be controlled. We extend the theory of these processes to include the electron-hole interaction. Our focus is on photon energies that excite carriers above the band edge, but close enough to it so that transition amplitudes based on low order expansions in $\mathbf{k}$ are applicable; both allowed-allowed and allowed-forbidden two-photon transition amplitudes are included. Analytic solutions are obtained using the effective mass theory of Wannier excitons; degenerate bands are accounted for, but envelope-hole coupling is neglected. We find a Coulomb enhancement of two-color coherent control process, and relate it to the Coulomb enhancements of one- and two-photon absorption. In addition, we find a frequency dependent phase shift in the dependence of photocurrent and spin current on the optical phases. The phase shift decreases monotonically from $\pi /2$ at the band edge to 0 over an energy range governed by the exciton binding energy. It is the difference between the partial wave phase shifts of the electron-hole envelope function reached by one- and two-photon pathways.

Near Threshold Photo-Oxidation of Dinucleotides Containing Purines upon 266 nm Nanosecond Laser Excitation. The Role of Base Stacking, Conformation, and Sequence

The 266 nm nanosecond laser photoionization (PI) of a wide range of dinucleotides containing purines, and related nucleosides and nucleotides, occurs through a combination of one- and two-photon pathways that can be considered as a near threshold PI process. The net PI yields of these DNA and RNA model compounds are of the order of 10-2, whereas the monophotonic component is of the order of 10-3. These yields increase with an increase in the initial ground-state concentration, which is interpreted in terms of a stabilization of their ionization potentials (IPs) due to an increase in the electronic coupling between the bases as the base stacking of the dinucleotides increases on going from 1 to 100 μM concentrations. Photodestruction yields for dinucleotides in the range 10-3 to 10-2 were determined using low-intensity 254 nm irradiation; these increased in the following order: GpC ≈ TpdG > CpG > ApG. Competition experiments using electron scavengers showed that the participation of the oxidized base in the photodestruction mechanism is 85% for GpC, 64% for TpdG, 59% for ApG, and 18% for CpG. The photoionization yields, estimated from the effect of electron scavengers on the photodestruction yield at low-intensity 254 nm irradiation, correlated well with the monophotonic PI yields of the dinucleotides examined. Differences in base stacking, conformation, and structural fluctuations of the various dinucleotides isomers are proposed to explain the sequence effect observed in the PI yields and photoreactivity at 254 nm. A linear correlation between the net PI yield of various dinucleotides and the calculated IPs reported by Saito and Rösch groups was obtained [Saito, I.; Nakamura, T.; Nakatani, K.; Yoshioka, Y.; Yamaguchi, K.; Sugiyama, H. J. Am. Chem. Soc. 1998, 120, 12686−12687; Voityuk, A. A.; Jortner, J.; Bixon, M.; Rösch, N. Chem. Phys. Lett. 2000, 324, 430−434]. Altogether, the results strongly suggest the use of near threshold PI yields of small oligonucleotides to estimate the effect of such properties as base stacking, sequence and conformation, base pairing, structural fluctuation, and solvation on the efficiency of hole-trapping and charge transport in DNA.

Photoionization of DNA and RNA bases, nucleosides and nucleotides through a combination of one- and two-photon pathways upon 266 nm nanosecond laser excitation.

The 266 nm nanosecond laser photolysis of various purine and pyrimidine derivatives results in their photoionization (PI) as one of the primary photochemical pathways. Electron photoejection occurs through a combination of one- and two-photon mechanisms. The PI values depend on the substituents attached to the chromophore of the base. The net PI of the purine bases at 266 nm are of the same order of magnitude (10(-2)) as those of the pyrimidine bases under similar experimental conditions. The monophotonic component is approximately one-third of the net PI yield of the bases. A nonrelaxed singlet excited state intermediate is tentatively proposed for this pathway. It is proposed that this state is significantly stabilized by water solvation, transforming it into a charge transfer to solvent state from which the hydrated electron evolves.

Transient solvent dynamics and incoherent control of photodissociation pathways in I2− cluster ions

Detailed time-resolved photodissociation and caging dynamics in clusters are studied using I2−(OCS)11 as a model system. We report new observations of product channel-dependent properties of nuclear coherence in the dissociated chromophore, reflecting complex dynamics of the solvent cage. The coherence feature is most pronounced in the caged two-photon channels and its relative amplitude increases with the product size. Shorter delays, on the time scale of coherent I⋯I− motion, favor larger products, allowing for incoherent control of two-photon dissociation pathways by appropriately timing the two laser pulses. As an example of such control, I2−(OCS)2 is produced most effectively by a limited set of pump–probe excitations at short delays. We emphasize generality of these results that relate to caging dynamics in any cluster ions.

HYDRATED ELECTRON FORMATION IN NANOSECOND and PICOSECOND LASER FLASH PHOTOLYSIS OF HEMATOPORPHYRIN IN AQUEOUS SOLUTION

Nanosecond (lambda exc = 266, 355 and 532 nm) and picosecond (lambda exc = 355 nm) laser flash photolysis of hematoporphyrin (Hp) was performed in neutral (pH 7.4) and alkaline (pH 12) aqueous solution, as well as in the presence of 0.1% Triton X-100. The dependence of the yield of photoproduced hydrated electrons (e-aq) on laser pulse energy was studied over a wide range of energies (0.2 to greater than 1000 mJ cm-2). The results show that e-aq are predominantly formed in a two-photon process at lambda exc = 266 and 355 nm. One-photon quantum yields are higher at lambda exc = 266 nm than at lambda exc = 355 nm. Both one-photon and two-photon pathways are less efficient at higher Hp concentration, reflecting the influence of Hp self-aggregation. Two-photon e-aq formation is more efficient when 30 ps pulses are used for excitation, as compared to 10 ns pulses. No e-aq could be detected at lambda exc = 532 nm. Nanosecond pulse-induced transient spectra obtained at pH 7.4 are also discussed.

Polarized absorption spectroscopy of Λ-doublet molecules: Transition moment vs electron density distribution

Polarized two-photon photodissociation of NO2 in the region of 480 nm yields NO(v″=0) with an anisotropic distribution of J. Measured polarization ratios are compared to quantum mechanical calculations for a range of expected ratios for the various isolated and mixed branches of the NO X̃2Π1/2→Ã 2Σ+ transition. Theoretical results show that main branches and their respective satellites (e.g., R11 and R21 branches) have the same transition moment directionally, though their intensities are in general different, implying that care is needed in interpreting polarization data from the mixed branches, such as (Q21+R11) or (Q11+P21), which measure the Π+ and Π− Λ doublet, respectively. Recognition of this fact is particularly important for properly separating the consideration of electron density distributions of Λ doublets from transition moment directionalities, as this has been a source of confusion in the literature. The measured results indicate that the principal two-photon photoexcitation pathway in NO2 photolysis is 2A1→1 2B2→2 2B2, with moderate A″ state mixing in the intermediate.

Photoexcitation of benzophenone triplets: a two-photon pathway for ground state repopulation

Room temperature laser excitation of benzophenone triplets in their visible absorption band in benzene solvent leads to their deactivation with concomitant ground state repopulation.