Laser Excited Biomolecules in Jet Expansions

YOUNGWOOD, Pa. - Sept. 16, 2014 - PRLog -- .
Background
In spectroscopy, biomolecules typically having extremely low vaporpressures and that decompose easily upon heating utilize jet expansion to bring the molecules into the gas phase. Collisions of expanding gases in the orifice of supersonic jets converts random thermal energy into translational energy that reduces the internal energy of the molecules by lowering their temperatures below 1 K. Cooling therefore enhances the quantum efficiency of laser excitation of the molecules by reducing non-radiative temperature losses. Indeed, extraordinary lifetimes of excited molecular states are observed [1] upon laser irradiation under the cooling of supersonic jet expansions.
But cooling by jet expansion is not limited to molecules alone. At the high-density region near the orifice, the cooling inducesthe condensation [2] of clusters. Upon laser excitation, the ionization of clusters in jet expansion produce excited states that at cooled temperatures enhances excited state spectroscopy.

Problem
Biomolecules in jet expansion cannot remain cool upon laser irradiation as the same heat capacity of constituent atoms that allowed the temperature to be lowered to < 1 K can only cause the temperature to increase upon heating by laser irradiation. Another mechanism is at play in the long lifetimes of molecules observed in jet expansion.

QM and QED Mechanism
Long-lifetimes of excited biomolecules is proposed to be the consequence of the QM heat capacity of the atom. QM stands for quantum mechanics. QM given by the Einstein-Hopf relation for the atom as a harmonic oscillator shows the heat capacity of a molecule may be described by the temperature and its EM confinement. EM stands for electromagnetic. The EM confinement depends on the external constraints placed on the atoms within the molecule, the constraints expressed in terms of the harmonic and anharmonic wavelength regions of the oscillator. At ambient temperature, the harmonic region of the oscillator has short oscillator wavelengths < 1 micron; whereas, the anharmonic region is characterized by long wavelengths > 30 microns. Generally, atoms in macroscopic structures have heat capacity, but in nanoscopic structures the heat capacity of the atom vanishes. See diverse QM applications at http://www.nanoqed.org

As the temperatures of jet expansion cool below ambient, the Einstein-Hopf relation shifts to higher wavelengths so as to increase the harmonic region of the QM oscillator at which the heat capacity vanishes. What this means is the temperature of molecular clusters in jet cooling cannot reach the micro K levels predicted from calculations based on the classical free expansions of gases.

The vanishing heat capacity required by QM not only affects the cooling temperature in jet cooling, but also the temperatures of the molecular clusters upon laser heating. Contrary to [3], molecules in jet expansions do not always increase in temperature upon laser irradiation, a conclusion that may be deduced by treating the molecule as a nanoscopic structure that by QM does not have the heat capacity to conserve the absorption of laser energy by an increase in temperature. Instead, the molecule conserves the laser energy by the creation of QED induced EM radiation that charges the atoms in the molecule, and if not, is emitted only to be absorbed by other molecules in the surroundings. QED stands for quantum electrodynamics. See QED applications, Ibid.

In the large spacing between molecular clusters in jet expansion, every cluster continually emits and absorbs QED radiation, but again, QM precludes any increase in temperature of the clusters. Only upon absorption of QED radiation by a macroscopic structure having heat capacity does the temperature increase. Traditionally, the increase in temperature of a gas is based on the assumption of the continuum where each molecule has heat capacity. But by QM, the temperature only increases if the QED radiation is absorbed by macroscopic entities having heat capacity such as large molecular clusters or the walls of the container.

SM differs from QM. SM stands for statistical mechanics. By SM, the heat capacity of an atom in a molecule never vanishes. SM implicitly assumes the EM confinement of the atom is always in the anharmonic region of the QM oscillator, i.e., there is no size effect in SM. Unlike QM, SM treats gas molecules in a container as having an average heat capacity which by QM is valid only in the anharmonic region under periodic boundary conditions.

The heat capacity of the atom is of importance in explaining the extended lifetime of excited states of biomolecules observed during laser excitation in jet expansions. In the figure above, the heat capacity of molecular clusters in jet expansion is depicted to decrease while the life-time increases with increasing spacing between molecules. The heat capacity of the molecule is related its excited state lifetime  as it controls the quantum efficiency by defining how much of the laser energy is converted to non-radiative losses from increases in temperature. In the QM interpretation, the heat capacity of the molecular cluster at large spacings vanishes, and therefore the EM energy from the laser is conserved with 100% quantum efficiency in a narrow absorption peak without non-radiative losses, the narrow absorption peak giving an extended lifetime by the uncertainty principle. Conversely, the uncertainty relation requires molecules having heat capacity to dissipate non-radiative losses over a broad absorption band require short-lifetimes. Simply put, long-lifetime excited states of biomolecules in jet expansion are a consequence of QM where the molecules have vanishing heat capacity while molecules with heat capacity have  short-excited lifetimes.

References
[1] M. Taherkhani. Development of a novel, long-lifetime supersonic jet source for laser spectroscopy of biological molecules, University of Manchester, PhD. Thesis, 2010
[2] O. F. Hagena and W. Obert. Cluster Formation in Expanding Supersonic Jets: Effect of Pressure, Temperature, Nozzle Size, and Test Gas, J. Chem. Phys.56, 1793, 1972.
[3] J. F. Ready, Effects of High-Power Laser Radiation, New York: Academic, 1971.