Edwin J. Heilweil

Pulsed terahertz spectroscopy of DNA, bovine serum albumin and collagen between 0.1 and 2.0 THz

We report the first use of pulsed terahertz spectroscopy to examine low-frequency collective vibrational modes of biomolecules. Broadband absorption increasing with frequency was observed for lyophilized powder samples of calf thymus DNA, bovine serum albumin and collagen in the 0.06–2.00 THz (2–67 cm−1) frequency range, suggesting that a large number of the low-frequency collective modes for these systems are IR active. Transmission measurements at room temperature showed increasing FIR absorption with hydration and denaturing.

Terahertz Spectroscopy of Short-Chain Polypeptides

Terahertz (THz) absorption and neutron vibrational spectra for solid-phase short-chain peptide sequences at 77 and 298 K are reported. We demonstrate that a high degree of spectral and structural information exists for these systems in the 1–15 THz (33–500 cm � 1 ) spectral range. The density and uniqueness of distinct spectral features for each system suggests that sequence-dependent structural information can potentially be extracted from these spectra using quantum mechanical solid-state modeling and theories. Published by Elsevier Science B.V.

Subpicosecond transient infrared spectroscopy of adsorbates. Vibrational dynamics of CO/Pt(111)

The vibrational dynamics of excited CO layers on Pt(111) were studied using infrared (IR) pump–probe methods. Resonant IR pulses of 0.7 ps duration strongly pumped the absorption line (ν≊2106 cm−1 ) of top‐site CO. Weak probe pulses delayed a time tD after the pump were reflected from the CO‐covered Pt(111) surface, and dispersed in a monochromator to determine the absorption spectrum of the vibrationally excited CO band, with time resolution <1 ps and monochromator resolution <1 cm−1. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for tD<0 show features characteristic of a perturbed free induction decay, which are expected based on multiple‐level density‐matrix models. For tD≥0, the CO/Pt absorption exhibits a shift to lower frequency and an asymmetric broadening which are strongly dependent on fluence (1.3–15 mJ/cm2 ). Spectra return to equilibrium (unexcited) values within a few picoseconds. These transient spectral shifts and the t...

Picosecond Time-Resolved Adsorbate Response to Substrate Heating: Spectroscopy and Dynamics of CO/Cu(100)

The response of the molecular stretch mode of CO/Cu(100) near 2086 cm−1 (ν1) to resonant infrared, and nonresonant visible and ultraviolet pumping is measured on a picosecond time scale. Fourier transform infrared measurements establish that ν1 is anharmonically coupled to the frustrated translation near 32 cm−1 (ν4), so that transient shifts in ν1 indicate population changes in ν4. The ν1 response to visible and ultraviolet pumping is characterized by a spectral shift near zero delay time, which decays with a ≊2 ps time constant to an intermediate value, which then decays on a ≊200 ps time scale. The data agree well with a model whereby ν4 couples to both the photogenerated hot electrons and to the heated phonons. The characteristic coupling times to these two heat baths are found to both be a few picoseconds.

Population relaxation of CO(v=1) vibrations in solution phase metal carbonyl complexes

Abstract Picosecond infrared saturation-recovery experiments were performed to obtain measurements of the vibrational energy lifetimes ( T 1 ) of CO( v = 1) vibrations ( v ≈ 1920-1985 cm −1 ) of carbonyl-containing metal complexes in dilute, room-temperature solutions. For relaxation of the F 1u CO-stretching vibration of W(CO) 6 in CCl 4 , CHCl 3 , n -hexane and benzene, T 1 , was found to be T 1 = 800±200, 480±50, 140±15 and 60±6 ps, respectively, while the same mode of Cr(CO) 6 , in these solvents gave T 1 = 440±70, 295±30, 145±25 and 59±6 ps. Monocarbonyl complexes with coordinated triphenylphosphine groups (TPP) have shorter CO( v = 1 ) lifetimes. These observations are rationalized in terms of molecular structure, intramolecular bonding, solvent interaction, and energy accepting vibrational modes.

Population lifetimes of OH(v=1) and OD(v=1) stretching vibrations of alcohols and silanols in dilute solution

Picosecond infrared pump–probe experiments determined the vibrational population lifetimes (T1) of the hydroxyl fundamental stretching mode OH(v=1) in 12 alcohols (R3COH) and 8 silanols (R3SiOH) in dilute room temperature CCl4 solutions. T1 for the silanols is in the range 185<T1<292 ps, while T1 for the alcohols is much less (T1<80 ps). The deuterium‐exchanged analogs (COD and SiOD) exhibit population relaxation times similar to protonated hydroxyls. An analysis of the vibrational energy levels corresponding to modes involving the four bonds nearest the hydroxyl groups of these molecules is used to qualitatively explain the trends of the observed T1 lifetimes for these systems. Solution T1 lifetimes are also compared to those previously measured for OH(v=1) on the surface of silica and in other condensed‐phase, room temperature systems.

Vibrational deactivation of surface OH chemisorbed on SiO2: Solvent effects

Picosecond infrared transmission spectroscopy was used to directly measure the vibrational energy relaxation time T1 of hydroxyl groups chemisorbed on the surface of colloidal silica (SiO2). T1 was obtained for OH(νstretch=1) in the strongly bound ‘‘isolated sites’’ of fumed silica particles in vacuum and dispersed in several liquids at T=293 K. At the SiO2/vacuum interface, T1=204±20 ps. When the SiO2 particles are surrounded by solvents, the relaxation time of the surface OH(v=1) groups decreases: for the liquids CCl4, CF2Br2, CH2Cl2, and C6H6, T1(ps)=159±16, 140±30, 102±20, and 87±30, respectively. T1 does not depend on the size of the SiO2 particles for the range 70 A≤ diameter ≤150 A, or on the surface OH coverage up to an average density of 4 OH/100 A2. Significant amounts of physisorbed water (5 H2O/100 A2) decreased T1 for the isolated OH(v=1) to T1=56±10 ps. For comparison to the surface hydroxyls, the vibrational deactivation time for OH(v=1) groups in the bulk of fused silica (OH/SiO2≊130 ppm b...

Ultrafast transient infrared absorption studies of M(CO)6 (M Cr, Mo or W) photoproducts in n-hexane solution

Abstract Following UV excitation, M(CO) 6 (MCr, Mo or W) species in solution lose one CO ligand, and a solvent molecule occupies the vacant site. These photochemical reactions are studied using a transient broadband IR absorption technique with 390 fs time resolution. The progress of these reactions, including loss of the M(CO) 6 reactant, appearance of a solvated M(CO) 5 ( n -hexane) product and vibrational energy distribution and relaxation of the product, is monitored through changes in infrared absorption by the CO ligands near 2000 cm −1 .

CO(v=1) population lifetimes of metal–carbonyl cluster compounds in dilute CHCl3 solution

Tunable infrared picosecond pulses in the 5 μ region have been used for time‐resolved pump–probe measurements of the population relaxation lifetime (T1) of CO(v=1) stretching vibrations in a series of metal–carbonyl cluster compounds in room temperature chloroform solution. T1 was the same for symmetric (ν=2084 cm−1, 90±10 ps) and antisymmetric (2014 cm−1, 87±10 ps) modes of the dicarbonyl Rh(CO)2(C5H7O2); T1 was the same for the B1 (2092 cm−1, 710±130 ps) and B2 (2036 cm−1, 750±90 ps) modes of Rh2(CO)4Cl2. Similarly long T1 times were found for Rh4(CO)12 (2075 cm−1, 610±65 ps) and Rh6(CO)16 (2077 cm−1, 700±100 ps). The molecule Co4(CO)12 has also been compared to the corresponding rhodium analog and it exhibits an initially fast relaxation of 47±5 ps followed by a slower 396±70 ps decay. The transient response of the more complex systems to the single frequency experiment is found to be sensitive to frequency and can exhibit bleaching, absorption, and a combination of these effects. Such behavior is attr...

Mercury cadmium telluride focal-plane array detection for mid-infrared Fourier-transform spectroscopic imaging

By combining step-scan Fourier-transform Michelson interferometry, an infrared microscope, and mercury cadmium telluride focal-plane array image detection we have constructed a mid-infrared spectroscopic imaging system that simultaneously records high-fidelity images and spectra of materials from 3500 to 900 cm(-1) (2.8 to 11 microm) at a variety of spectral resolutions. The fidelity of the spectral images is determined by the pixel number density of the focal-plane array. Step-scan imaging principles and instrument design details are outlined. Spatial resolution measurements and infrared chemical imaging examples are presented, and the results are discussed with respect to implications for chemical analysis of biosystems and composite materials.