Showing papers on "Absorption spectroscopy published in 1981"

Theoretical results concerning light absorption in a discrete medium, and application to specific absorption of phytoplankton

Abstract: The radiation absorption within a medium supposedly non-absorbing in itself but containing an absorbing substance as particles is examined theoretically. Absorption by such particles can be treated independently of (Mie) scattering under an assumption concerning the complex refractive index that is reasonable in case of algal cells (index close to that of water and weakly absorbing substance). The effect of discreteness on absorption properties is ruled by a function of the dimensionless parameter that combines through their product the size (d) and the absorption coefficient of the cell material (acm). The limiting value of this function (d → 0) describes the case of a true solution of the same material, whereas its variations with d and acm imply that the specific absorption coefficient, for a given substance, is variable in magnitude and in spectral behaviour. Consequently, Beer's law, which rests on the existence of a constant specific coefficient, generally cannot apply when canopy changes intervene in an algal population. Various cases of such changes are studied along with their consequences on absorptive properties. The theoretical conclusions are exemplified by some experimental results concerning algal cultures. The absorption properties of the cell material for each species can be extrapolated from the actual absorption spectra of intact cells. Problems that originate from the non-constancy of the specific absorption coefficient (spectral values or mean value) are examined in view of two kinds of applications: the photosynthetic (quantum) yield evaluation and the algal biomass assessment by remote sensing, for which the constancy is generally postulated.

Quantum size effect in three-dimensional microscopic semiconductor crystals

Abstract: The exciton absorption spectrum of microscopic CuCl crystals grown in a transparent dielectric matrix has been studied The size of the microscopic crystals was varied in a controlled manner from several tens of angstroms to hundreds of angstroms There is a short-wave shift (of up to 01 eV) of the exciton absorption lines, caused by a quantum size effect

Vibrational interaction between molecules adsorbed on a metal surface: The dipole-dipole interaction

Abstract: We have developed a theory for the properties of vibrational excitations in molecules adsorbed on a metal surface. The coherent potential approximation (CPA) is used in the treatment of the vibrational interaction between the molecules. We show, by interpreting infrared spectra of substitutionally disordered systems consisting of isotopic mixtures of CO on Cu(100), that the molecules interact mainly through their dipole fields. We also show that in interpreting the integrated absorptance in infrared spectroscopy or the relative loss intensity in electron-energy-loss spectroscopy it is necessary to take into account the screening due to the electronic polarizability of the adsorbed molecules. A simplified version of the CPA result is used for a discussion of the absorption spectra of partial monolayers of one isotope. With the assumption that the CO molecules are randomly distributed, comparison between theory and experiment indicates that the dipole-dipole interaction alone is responsible for the coverage-dependent frequency shift for CO adsorbed on a transition metal [Ru(001)], whereas there is an almost equally large counteracting chemical shift on a noble metal [Cu(100)]. The meaning and origin of the dynamical dipole moment of adsorbed CO molecules are discussed. We find that the increase of the dynamical dipole moment (by a factor 2-3) upon adsorption probably is due to charge oscillations between CO $2{\ensuremath{\pi}}^{*}$ molecular orbitals and the metal. Finally, we outline how the theory developed here can be applied to a fundamental step in photosynthesis.

Spectral dependence of the change in refractive index due to carrier injection in GaAs lasers

Abstract: The refractive index change caused by changes in the absorption edge of GaAs is determined by analysis of the spontaneous emission spectrum of a buried heterostructure window laser. The spontaneous emission spectrum is converted to a gain spectrum from which changes in the imaginary part of the refractive index can be calculated as the laser is excited from low current up to threshold. The real change in refractive index is then determined by a Kramers‐Kronig transformation. The change in refractive index n′ of the GaAs active layer is slightly sublinear with minority carrier density nc. At the laser line, including the calculated contribution of free carriers, Δn′ = −0.025±0.005 and dn′/dnc = −(1.8±0.4)×10−20 cm3 in lasers with carrier densities at threshold estimated as 1.02×1018 cm−3. Near threshold, the ratio of the change in the real index to the change in the imaginary index is about 6.2.

Near-infrared spectral reflectance of mineral mixtures - Systematic combinations of pyroxenes, olivine, and iron oxides

Abstract: Near-infrared spectral reflectance data are presented for systematic variations in weight percent of two component mixtures of ferromagnesium and iron oxide minerals used to study the dark materials on Mars. Olivine spectral features are greatly reduced in contrast by admixture of other phases but remain distinctive even for low olivine contents. Clinopyroxene and orthopyroxene mixtures show resolved pyroxene absorptions near 2 microns. Limonite greatly modifies pyroxene and olivine reflectance, but does not fully eliminate distinctive spectral characteristics. Using only spectral data in the 1 micron region, it is difficult to differentiate orthopyroxene and limonite in a mixture. All composite mineral absorptions were either weaker than or intermediate in strength to the end-member absorptions and have bandwidths greater than or equal to those for the end members. In general, spectral properties in an intimate mixture combine in a complex, nonadditive manner, with features demonstrating a regular but usually nonlinear variation as a function of end-member phase proportions.

The infrared spectra of amorphous solid water and ice Ic between 10 and 140 K

Abstract: Absorption spectra from 4000 to 1200 cm −1 of amorphous solid water and polycrystalline ice I c have been measured between 10 K and 140 K. Warm up and recooling of an H 2 O sample prepared at 10 K gives rise to both irreversible and reversible changes in the peak frequency, band width, and peak height as well as the integrated intensity of the OH stretching band. These spectral effects are related to structural differences. The structure of amorphous solid water also depends on deposition conditions. The optical constants of amorphous so water are determined at 10 K and 80 K from a Kramers-Kronig analysis of the transmission spectra taking into account reflection and interference losses. The astrophysical implication of the temperature dependence of peak frequency and band width of the 3250 cm −1 band in amorphous solid water is discussed briefly.

Electronically excited and ionized states of the chlorine molecule

Abstract: Potentials curves for the ground and excited states of the chlorine molecules and its positive and negative ions have been calculated by means of the MRD-CI method. The standard AO basis employed consists of 74 functions including two atomic d and one set of s and p bond species, and the results at the corresponding full CI level are estimated for each state via a perturbation correction. Special emphasis is placed upon the treatment of Rydberg-valence mixing in this system, which phenomenon is found to be essential to the understanding of Cl2 electronic absorption spectrum. All singlet states which correlate with the lowest dissociation limit plus many others which go to ionic Cl++Cl− or Rydberg Cl+Cl asymptotes are given explicit consideration. Among the triplet species of Cl2 which dissociate into the ground state atoms only the 3Πu state is not repulsive. The calculated D0 value for the ground state is 2.455 eV compared to the experimental value of 2.475 eV, while the vertical ionization energy and electron affinity are found to be 11.48 and 2.38 eV respectively, also in very good agreement with the corresponding measured data of 11.50 and 2.51 ± 0.1 eV. In addition to Cl2 laser line is confirmed to result from a 3Πg → 3Πu emission, whereby the calculated downward vertical transition energy of 4.86 eV fits in quite well with the known location of this line at 4.805 eV. The first two dipole-allowed transitions from the ground state of chlorine involve 1Σu+ and 1Πu states which are calculated to be nearly isoenergetic, and these results also match very well with the location of the first absorption band in this spectrum. Finally quite similarly as in O2 it is found that an avoided crossing between Rydberg and valences states produces a relatively steep potential well for an upper state (2 1Σu+), whose location concides with that of a second absorption band recently observed in synchrotron radiation studies.

Absorption Spectrum of DNA for Wavelengths Greater than 300 nm

Abstract: Although DNA absorption at wavelengths greater than 300 nm is much weaker than that at shorter wavelengths, this absorption seems to be responsible for much of the biological damage caused by solar radiation of wavelengths less than 320 nm. Accurate measurement of the absorption spectrum of DNA above 300 nm is complicated by turbidity characteristic of concentrated solutions of DNA. We have measured the absorption spectra of DNA from calf thymus, Clostridium perfringens, Escherichia coli, Micrococcus luteus, salmon testis, and human placenta using procedures which separate optical density due to true absorption from that due to turbidity. Above 300 nm, the relative absorption of DNA increases as a function of guanine-cytosine content, presumably because the absorption of guanine is much greater than the absorption of adenine at these wavelengths. This result suggests that the photophysical processes which follow absorption of a long-wavelength photon may, on the average, differ from those induced by shorter-wavelength photons. It may also explain the lower quantum yield for the killing of cells by wavelengths above 300 nm compared to that by shorter wavelengths.

Evidence for sulphur implantation in Europa's UV absorption band

Abstract: The UV spectral characteristics of the Galilean satellites are investigated (using data from the International Ultraviolet Explorer (IUE) spacecraft) as a function of the orbital position, large-scale areal variability, and temporal dynamics. The discovery of an absorption feature at 280 nm in Europa's reflection spectrum is reported and observations show that the absorption is strongest on the trailing hemisphere (central longitude 270 degrees). The feature resembles SO2 and seems to result from S-O bond formation between deeply implanted sulphur atoms and the adjacent damaged water-ice-lattice. The sulphur supposedly comes from energetic (hundreds of keV) sulphur ions that are present in the Jovian magnetosphere. An appropriate equilibrium condition can be found to match the observed spectral data if sputtering erosion occurs at no greater than approximately 20 meters per one billion years.

Polarized absorption and linear dichroism spectroscopy of hemoglobin.

Abstract: Publisher Summary This chapter is concerned with polarized absorption and linear dichroism spectroscopy of hemoglobin. Polarized absorption and linear dichroism are techniques that are used to study the optical properties of oriented systems. Unlike solutions, where light polarized in any direction is absorbed equally because the molecules are randomly oriented, the absorption of plane-polarized light by oriented molecules is dependent on the polarization direction of the incident light beam. Anisotropic absorption occurs because molecules fixed in space exhibit maximum absorption when the electric vector of the light is parallel to well-defined directions in the molecule. A polarized absorption experiment in which the spectra are measured in each of three orthogonal directions on a sample of known molecular orientation—such as a crystal of known structure—yields both the complete spectrum and the molecular direction of the transition moment for each absorption band. If, on the other hand, the transition moment directions are already established, the polarized absorption experiment along three orthogonal axes yields the molecular orientation. In a linear dichroism experiment, the difference between the optical densities in two directions is measured directly.

Water frost and ice: The near-infrared spectral reflectance 0.65–2.5 μm

Abstract: The spectral reflectance of water frost and frost on ice as a function of temperature and grain size is presented with 1-1/2% spectral resolution in the 0.65- to 2.5-micron wavelength region. The well-known 2.0-, 1.65-, and 1.5-micron solid water absorption bands are precisely defined along with the little studied 1.25-micron band and the previously unidentified (in reflectance) 1.04-, 0.90-, and 0.81-micron absorption bands. The 1.5-microns band complex is quantitatively analyzed using a nonlinear least squares algorithm to resolve the band into four Gaussian components as a function of grain size and temperature. It is found that the 1.65-micron component, which was thought to be a good temperature sensor, is highly grain-size dependent and poorly suited to temperature sensing. Another Gaussian component appears to show a dependence of width on grain size while being independent of temperature. The relative apparent band depths are different for frost layers on ice than for thick layers of frost and may explain the apparent band depths seen in many planetary reflectance spectra.

Improved Data of Solar Spectral Irradiance from 0.33 to 1.25 μ

Abstract: The conversion of our centre of disk intensities published in 1968/70 into mean disk intensities has been repeated, using more accurate data for the centre-to-limb variation of both continuous radiation and strong absorption lines.

Theoretical Studies of Charged Defect States in Doped Polyacetylene and Polyparaphenylene

Abstract: Defect statecalculations have been per formed for polyacetylene and polybparaphenylene in the fraamework of the Su, schriever, and Heeger Hamiltonian. In polyacetylene, the study of the energetics of the separation of the radical(natural defect)-ion(charged defect) pair induced upon doping indicates that the two defects tend to remain close to each other. This results in the formatn of polarons whose binding energy is estimated to be of the order of 0.05 eV Absorption spectra at low doping levels are consistent with polaron formation. Interaction between polarons leads to the formation of charged solitions. In poly(p-phenylene), defects are always correlated in pairs. Upn doping, polarons are formed (binding energy ∼ 0.03 eV), wit the relaxation of the lattice extending over about four rings. Calculations suggest the possibility of bipolarons (doubly charged defects) that yield conductivity without Pauli susceptibility.

A Fast X-Ray Absorption Spectrometer for Use with Synchrotron Radiation

Abstract: A quasi-parallel and polychromatic beam of synchrotron radiation is focused and dispersed by a curved crystal, so that the energy of each ray of the focused beam varies as a function of convergence angle through the focus. The specimen is placed at the focus. By measuring the X-ray intensity distribution across the beam behind the focus, in the presence and absence of the specimen, the absorption spectra of Cu and Ni metal foils were obtained. Using an X-ray film as the detector, a spectrum from a Cu foil was obtained in 0.1 seconds when the SPEAR storage ring at Stanford was operated at 3.1 GeV and 80 mA. The energy resolution is approximately 2.0 eV and the energy range of the spectrum is approximately 1 keV.

Two-mirror multipass absorption cell.

Abstract: A multipass cell for absorption measurements with laser light is described. The number of passes is adjusted by variation of the distance of two parallel concave mirrors. The position and direction of the exit beam do not change when the path length is varied. A cell of 1-m length was used for infrared absorption measurements with an effective path length between 3 and 150 m.

Spectroscopy and photokinetics of chromium (III) in glass

Abstract: The optical absorption, emission, emission lifetime, and absolute quantum efficiency have been measured for chromium (III) in a wide variety of oxide and fluoride glasses. In all cases, the room temperature emission is dominated by inhomogeneously broadened 4T2 fluorescence, and the relaxation of this level is shown to be primarily nonradiative. Multiphonon emission to lattice modes, concentration quenching, and quenching by hydroxyl moieties is shown not to be of significance in this relaxation at low doping levels. Instead, it is proposed that the radiationless process involves primarily local modes associated with the CrX6 cluster and is sensitive to site symmetry.

Interband Absorption Spectra of Disordered Semiconductors in the Coherent Potential Approximation

Abstract: The interband absorption spectra of a system with Gaussian site diagonal disorder are calculated in the coherent potential approximation. The electron-hole vertex part due to interband stochastic correlation is taken into account. The relaxation of the k -selection rule and the effect of localization are studied through the calculation of the optical transition matrix elements. It is clarified that the band tail states contribute to an exponential absorption tail (the so-called Urbach tail) as a result of interplay between transfer energy and Gaussian-distributed site energies. Introduction of a negative interband correlation leads to a red shift of the absorption edge. This effect is discussed in connection with the photo-structural change observed in chalcogenide glasses.

Near Infra-Red Absorption Spectra of the Chlorophyll a Cations and Triplet State in vitro and in vivo

Abstract: Absorption spectra in the near infra-red have been obtained for transient species obtained from chlorophyll a, by flash absorption spectroscopy. The triplet state of Chl a presents an absorption band around 760 nm (ϵ = 7,500 M−1 cm−1) and a very broad band (ϵ = 1,700 M−1 cm−1) around 1100 nm. At low temperature, evidence was obtained for the formation of the triplet state of aggregated Chl a. Its t1/2 of decay is around 0.5 ms instead of 1 ms for the monomeric species. The radical-cation Chl a+ was formed by reaction of 3Chl a with a quinone; it has an absorption band at 840 nm (in cyclohexanol) with a shoulder around 740 nm and no absorption between 1000 and 1650 nm. The cation has also been formed in vivo, as the oxidized state of the photosynthetic primary electron donors, P680 and P700. The spectrum of P+680 resembles that of Chl a+; its absorption maximum is at 820 nm. The spectrum of P+680, maximum at 810 nm, is significantly broader. P+700 has no absorption between 1000 and 1650 nm. These spectra are in favor of recent suggestions on a monomeric nature of P680 and of a dimeric nature for P700.

Near-edge X-ray absorption spectra for metallic Cu and Mn

Abstract: The measurement of X-ray absorption fine structure of metals— both in the extended region (EXAFS) as well as in the near edge region (XANES)—has been widely discussed (see refs 1–6 for Cu and refs 7–9 for Mn). The recent availability of intense X-ray fluxes from storage rings has usually been exploited for EXAFS leaving the XANES often with poorer resolution than earlier work performed on conventional sources (for example, compare the near edge structure for copper in ref. 1 with refs 3 or 6). In addition, whilst the theory and analysis of EXAFS is relatively well-established2,10, a theory for the strong scattering regime near to the absorption edge has only recently been developed11. We report here the first high resolution XANES spectra for Cu and Mn which were performed at the SRS storage ring at Daresbury. Although both metals have close-packed structures consisting of atoms of similar size their local atomic structure is different in detail. Significant differences are found in their respective XANES reflecting the senstivity of this region of the X-ray absorption fine structure to the local atomic structure. Spectra for the two metals have been analysed using the new multiple scattering formalism. This is a real space calculation and unlike a conventional band structure approach it does not require structural periodicity but works from the local arrangement of atoms.

Optical properties of Nd3+ in metaphosphate glasses

Abstract: Absorption and fluorescene spectra and fluorescence lifetimes of Nd 3+ ions were measured for the following polymeric metaphosphate glasses: [ M (PO 3 ) 2 ] n , where M = Mg, Ca, Sr, Ba, Zn, Cd, and Al(PO 3 ) 3 . Judd-Ofelt intensity parameters for f-f transitions were derived from the integrated absorption spectra and used to calculate the spontaneous emission probabilities from the 4 F 3 2 state. Metaphosphate glasses exhibit systematic variations of refractive indices, optical intensity parameters, fluorescence lifetimes and linewidths, and stimulated emission cross sections with alkaline earth. The origin of these variations and their implications for tailoring spectroscopic properties by compositional changes are discussed. Neodymium laser action in metaphosphate glasses is also considered.

Characterization of bacteriochlorophyll interactions in vitro by resonance Raman spectroscopy

Abstract: The effect of several types of BChl interactions of its resonance Raman (RR) spectrum has been examined. In monomeric BChl species, ligation interactions at the central Mg atom produce significant changes in its spectrum and two structure-sensitive bands are identified, which are analogous to those in other porphyrins. Hydrogen-bonding interactions in monomeric BChl affect only the C = O modes, which are weakly in resonance at the excitation wavelength used here. In aggregated BChl species, large differences are observed in the RR spectra as compared to monomeric BChl for two of the three types of aggregates studied. These are the BChl pyrazine adduct (bifunctional ligand aggregate) and the BChl hydrate (hydrogen-bonded aggregate) absorbing at 845 nm. Both of these aggregates display RR spectra which are distinct from one another as well. The third type of BChl aggregate, that formed through coordination interactions of the acetyl or keto carbonyl group of one molecule with the Mg atom of another (self-aggregate), exhibits a spectrum which differs from monomeric BChl only in relative band intensities; no frequency shifts are observed. Additional results which are discussed include the identification of metal-sensitive and deuteration-sensitive bands. A comparison of the RR spectra of BChl a and its metal-free analogue, BPheo a, shows there are three bands which are sensitive to metalation in the region 1000-1800 cm-’. The fully deuterated spectrum of BChl is substantially different from the fully protonated. Researchers in the field of photosynthesis have long recognized the need for determining the molecular organization in the photosynthetic membrane in order to understand the processes of energy transfer among the light-harvesting components and photoinduced charge separation in the reaction center. Electronic absorption spectroscopy has been used extensively to study the chlorophylls, which play a major role in both of these processes.’J The popularity of this technique results not only from its experimental simplicity but also because the chlorophylls have intense electronic transitions in the visible region of the electromagnetic spectrum which are significantly perturbed in the intact organism. Large shifts to lower energy are observed in the red transitions of chlorophyll (Chl) in green plants and bacteriochlorphyll (BChl) in photosynthetic bacteria relative to the extracted pigments in organic solvent^.^*^ The shifts are most pronounced in the case of BChl-containing organisms. For example, antenna BChl in Chromatium vinosum has an absorption maxima at 890 nm or approximately 120 nm to the red of that for BChl in diethyl ether. Several theories have evolved concerning the nature of Chl and BChl interactions in vivo which purport to explain the red shift in their absorption maxima. The two most common include (1) noncovalent interactions of the pigment molecules with proteins4 and (2) aggregation (Chl) or hydration (BChl) of the pigments.’J These interactions are thought to result in a lowering of the electronic transition energy through medium effects, donor-acceptor interactions, exciton interactions, or some combination of the aforementioned. In the case of the first theory, there is strong experimental evidence to support chlorophyll-protein interactions from disk gel analysis of the photosynthetic membrane proteins! In many cases almost 100% of the pigment can be accounted for in the various protein bands if mild detergent treatments are used to dissociate the membranes. Also, X-ray crystallographic analysis of the BChl-protein complex from Prosthecochloris aestuarii has shown that the seven bacteriochlorophyll a molecules of this complex are contained within a protein matrix. It appears that functional groups on the protein side chains are coordinated to the Mg atoms of the BChl molecules. With respect to the second theory, in vitro studies of model systems have shown that sizable red shifts in the chlorophyll absorption maxima are achieved only in cases where there are chlorophyll-chlorophyll interactions.l8 In Chl a, self-aggregation causes shifts on the order of those observed in vivo.’ In BChl a, various polymeric hydrates in nonpolar solvents exhibit absorption maxima analogous to those observed in bacterial antenna complexes.lb Thus far there have *Address correspondence to this author at Northwestern University. been no reports of in vivo or synthetic chlorophyll-protein complexes which clearly contain monomeric chlorophyll and exhibit sizable red shifts in the chlorophyll absorption spectruma6 Although such negative evidence is not conclusive, it does suggest that perhaps the absorption properties of the chlorophylls in vivo are a result of both chlorophyll-chlorophyll and chlorophyllprotein interactions. There are obvious limitations to the use of absorption spectroscopy for the study of chlorophyll interactions. Broad absorption bands do not contain structural information and cannot, for that matter, be correlated with unique chlorophyll associations. Other spectroscopic techniques which have been used to define chlorophyll interactions in solution such as N M R and IR cannot be used in the traditional mode to study molecular interactions in complex biological membranes or large protein molecules. One of the most promising spectroscopic approaches for studying chromophorecontaining systems is resonance Raman spectroscopy (RRS), which employs laser excitation frequencies in, or near, resonance with an electronic transition in the chromophore-bearing system. There are a t least two main advantages to R R spectroscopy. First, the molecular vibrations of the chromophore are preferentially enhanced over those of its environment when the laser wavelength is in resonance only with the chromophore. Second, the vibrational spectrum of the chromophore which results is simpler than its normal Raman (NR) spectrum because only those modes which are coupled to the electronic transition are resonance enhanced. The RR spectra, therefore, can also be used to determine information about the excited state of the molecule by monitoring vibrational intensities as a function of excitation (1) (a) Katz, J. J.; Shipman, L. L.; Cotton, T. M.; Janson, T. R. “The Porphyrins”; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 5, Part C, pp 401-458 and references therein. (b) Katz, J. J.; Oettmeier, W.; Norris, J. R. Philos. Trans. R. SOC. London, Ser. B 1976, 273, 227-253. (2) Gcedheer, J. C. “The Chlorophylls”; Vernon, L. P., Seely, G. R., Us.; Academic Press: New York, 1966; pp 147-184 and references therein. (3) (a) Sauer, K. In “Bioenergetics of Photosynthesis”; Govindjee, Ed.; Academic Press: New York, 1975; pp 115-181. (b) Brown, J. S. Annu. Rev. Plant Physiol. 1972, 23, 73-86. (4) Thornber, J. P.; Trosper, T. L.; Strouse, C. E. In “Photosynthetic Bacteria”; Clayton, R. K., Sistrom, W. R., Eds.; Plenum Press: New York, 1978; pp 133-160 and references therein. (5) Matthews, B. W.; Fenna, R. E. Acc. Chem. Res. 1980, 13, 309-317 and references therein. (6) Synthetic Chl a derivative complexes with myoglobin have been reported by two groups, but neither complex exhibits a substantial red shift in the complex relative to that of the isolated chromphore. See the following: (a) Boxer, S. G.; Wright, K. A. J . Am. Chem. SOC. 1979,101,6791-6794. (b) Davis, R. C.; Pearlstein, R. M. Nature (London) 1979, 280, 413-415. 0002-7863/81/lS03-6020$01.25/0

Dimeric and trimeric states of the fluorescein dianion. Part 1.—Molecular structures

Abstract: The formation constant and absorption spectrum of the dimer and trimer of fluorescein dianion in aqueous solution are evaluated. The geometric structures of both aggregates are determined using the exciton theory. The nature of the association forces is also studied.

Quantum molecular dynamics in intense laser fields: Theory and applications to diatomic molecules

Abstract: The quantum dynamics of vibration–rotation excitation in diatomic molecules in intense laser fields is investigated. The Floquet method is used in solving the equations‐of‐motion for a Hamiltonian explicitly time‐dependent. This method requires computation of the time‐displacement propagator only over the first optical cycle of the laser field, and is accomplished both numerically and with Magnus approximations. A number of features of single and multiphoton absorption in the LiH, CO, IBr, and HF molecules are studied as functions of the laser intensity and frequency. Average photon absorption spectra are studied with respect to power broadening, dynamic Stark shifts and line shapes. In some cases effective two‐state perturbative results accurately agree with the numerical results. In addition, rotational distributions in IBr following two‐photon absorption are found to have both thermal components for nonresonant states and structured nonthermal components for states nearly in resonance with the field. Finally, the intensity and temperature dependence of a two‐photon laser induced isotopic enrichment in CO is studied. The 14CO to 13CO enrichment ratio is found to decline with increasing laser intensity and initial rotational temperature.