MEANS AND METHOD TO PROVIDE LASING

Abstract: For producing vacuum ultraviolet (VUV) radiation, a laser apparatus, (Fig. 1) for inverting a population in an atomic or molecular species is provided. Energy is stored in a first species, (50) and a laser induced collisional energy transfer from that first species, (50) to a selected dis-sociative molecular state of a second species, (51) causes an inversion of one of the constituents of the second species, (51). This is represented, in one embodiment, (Fig. 1) of the present invention which utilizes a second species, (51) comprising a diatomic molecule, by the reaction. (FORMULA) where A and A* are the ground and excited states of the first species, (50) denoted as the storage species; (BC) and (BC)* are the ground and excited states of the second species, (51) which comprises atoms B and C, and B* is the excited state of atom B, which excited state is inverted.

Description: MEANS AND METHOD TO PROVIDE LASING Background of the Invention This invention pertains to the field of molecular dissociation lasers and more particularly to the field of molecular dissociation lasers which are pumped by laser induced collisions. The use of molecular photodissociation as a means for producing photo-fragments in selected excited states has been summarized in a review paper entitled, "Quantum Efficiency of Florescence Excited by Photodissociation in Metal Halide Vapors and Applications", IEEE Journal of Quantum Electronics, Vol. QE-15, No. 7, July 1979, Pp. 579-594, by Jakob Maya. A wide variety of atomic resonance line and molecular lasers have been fabricated using selective photodissociation as the inversion mechanism. However, the availability of practicable pump lasers has limited the choice of target molecules to those which absorb and dissociate at wavelengths of -1900A or greater. These systems have generally yielded lasers in the visible or near-inf ared and the extension of photodissociation techniques to other molecular systems to produce new vacuum ultraviolet (VϋV) laser sources has proven difficult due to the lack of suitable pump sources at wavelengths shorter than 1900A. Summary of the Invention A laser apparatus for inverting a population in an atomic or molecular species is based on selective energy storage followed by the energy transfer mechanism of laser induced collisions. Energy is stored in a first species and a laser induced collisional energy transfer from that first species to a selected dissociative molecular state of a second species causes an inversion of one of the constituents of the second species. This is represented, in one embodiment of the present invention which utilizes a second species comprising a diatomic molecule, by the reaction A* + BC+hv-> A + (BC)* -> A + B* +C (1) where A and A* are the ground and excited states of the first species respectively, denoted as the storage specie BC and (BC) * are the ground and excited states of the second species respectively, which second species compris atoms B and C, and B* is the excited state of atom B, whi excited state is inverted. In this embodiment, energy is stored in storage species A by exciting an atom in the ground state to an excited state A*. A photon of energy hv, applied from laser source, excites A* to a virtual level. During a collision of A* with molecule BC, a dipole-dipole interaction between the two species results in the de- excitation of A* to the ground state and the excitation o BC to a dissociative molecular state. The dissociative molecular state is chosen so that B* is formed in the upp level of a laser transition or rapidly decays thereto, whereby the population of that laser transition is inverted. Brief Description of the Drawing FIG. 1 shows a laser apparatus according to th present invention. FIG. 2 shows the energy level diagraπ corresponding to a first embodiment of the present invention for providing a deuterium Lyman α laser; and FIG. 3 shows the energy level diagram corresponding to a second embodiment of the present o invention for providing an atomic Iodine 1583A laser. - 3 - Detailed Description A laser apparatus for inverting a population in an atomic or molecular species is based on selective energy storage followed by the energy transfer mechanism of laser induced collisions. Energy is stored in a first species and a laser induced collisional energy transfer from that first species to a selected dissociative molecular state of a second species causes an inversion of one of the constituents of the second species. This is represented, in one embodiment of the present invention which utilizes a second species comprising a diatomic molecule, by the reaction A* + BC + h.v -> A + (BC) -> A+B* +C (1) where A and A* are the ground and excited states of the first species respectively, denoted as the storage species, and BC and (BC) are the ground and excited states of the second species respectively, which second species comprises atoms B and C, and B* is the excited state of atom B, which excited state is inverted. In this example, energy is stored in storage species A by exciting an atom in the ground state to an excited state A*. A photon of energy / applied from a laser source, excites A* to a virtual level. During a collision of A* with molecule BC, a dipole-dipole interaction between the two species results in the de- excitation of A* to the ground state and the excitation of BC to a dissociative molecular state (BC)*. The dissociative molecular state, (BC)*, dissociates into B* and C, whereby the population of atom B Δ - in the excited state B* or an excited state to which B* rapidly decays is inverted. Thus, a laser transition may take place between an excited state of B and a lower stat of atom B. By choosing an appropriate excited state of storage species A, one can store energy slowly in the excited state of storage species A. The collisional cros section for energy transfer between the ground state of molecule BC and the excited state of A* is quite low when the energy level of an excited state of BC is not comparable in energy to the energy level of the metastabl state A*. However, when a photon of energy hv is applie to A*, which energy, when added to the energy of the metastable state A*, is close in value to the energy leve of (BC)*, the collisional cross section for energy transf between A* and BC is made large. Thus, an important characteristic of the use of a laser induced collisional energy transfer is the ability to store energy slowly in storage species A and then rapidly to transfer this energ to the desired molecular state of the second species. Th characteristic provides an advantage for inverting VUV laser transitions, which transitions typically have radiative lifetimes of a few nanoseconds or less. In addition, the use of tunable laser source for providing t photons which induce the collisional energy transfer allo pumping of molecular states of the second species which might otherwise be inaccessible to fixed frequency laser sources. Although we have discussed the present inventio with an example having an atomic storage species, we note that, in general, the storage species may be a molecule a well as an atom. Also, in the most general aspect of the present invention, the excitation mechanism for exiting t storage species from the ground state to the desired metastable excited state may be accomplished by a variety of mechanisms as will be clear to those skilled in the ar among these being electron excitation in a discharge tube an electron beam, or laser pumping. Finally, as noted above, the second species need not be merely a diatomic molecule. In fact, the dissociative state may comprise excited molecular fragments. FIG. 1 shows an apparatus constructed according to an embodiment of the present invention. Arrows 50 and 51 indicate the flow of the storage species and second species, where both species are gaseous, into laser cavity 3. Alternatively, the two species can be introduced into the cavity in solid or liquid form and thereafter treated, e.g., heated, to convert them to gases. Also, it is theoretically possible for the process to be performed using the species in liquid form. Laser source of radiation 1 provides laser beam 110. Laser beam 110 is directed into laser cavity 3 by reflector 4. Radiation from laser beam 110 is used to excite the storage species to the appropriate excited state. Laser source of radiation 2 provides laser beam 100. Laser beam 100 is directed into laser cavity 3 by reflector 5. Radiation from laser 100 induces the collisional energy transfer from the excited state of the storage species to the excited state of the second species. The following describes embodiments of two new VUV lasers which are constructed according to the present invention. In a first embodiment the level of the second species which is excited by the laser induced collisional energy transfer is a bound state, which bound state is strongly predissociated to yield the desired excited fragments. In the second embodiment, the level of the second species which is excited is unbound, i.e. repulsive at small internuclear separation and therefore exhibits a broad absorption for forming the desired excited fragments. The embodiments described hereinbelow provide storage of energy in a metastable excited state of the storage species by means of selective optical pumping. _ OMPI The first embodiment of the present invention disclosed herein for energy transfer to a strongly predissociated molecular state is described by: (E,F λ ∑g) + HD(X λ ∑ g ) + hv(_6650A) -> H 2 (X 1 ∑ τ g ~ ) ' A + m HD**' ( n D ! " ' -> H,(X ' ∑ ) + H(n=l) + D*(n = 2) (2) An energy level diagram corresponding to the reaction described by EQ. 2 is shown in FIG. 2. First, energy is stored in the B. (E,F ^ ∑ ) state of the hydrog molecule, the storage species in this embodiment, by two photon pumping of the X ∑ ground state of a hydrogen o ~ molecule with 1930A radiation from an ArF excimer laser. o Laser radiation from a tunable "transfer laser" at -6650A is applied to the H . The HS may be viewed as being excited to a virtual level, which virtual level is couple to the B" ∑ u and D ^ π u states of the hydrogen molecule While in this virtual level, the \_* molecule collides wit X Σ ground state of a HD molecule and undergoes a dipole-dipole interaction. This results in the de- excitation of the HS molecule to the ground state and the excitation of the HD molecule to the D π u state. As is shown in an article entitled, "H(2s) and H(2p) Branching Ratio in the Photodissociation of H 2 Near Threshold", Journal of Chemical Physics, Vol. 67, No. 9, November 1, 1977, pp. 3845-3850, by j . E. Mentall and P. Y. Guyon, the D π u level in H 2 is strongly predissociated by the B" ^ ∑ u state to yield an excited ' state H* (n=2) atom and a ground state H(n=l) atom with nearly unity quantum efficiency. In HD the predissociati of the D π u level yields...pro.duct states H(n=l) and D* (n=2) . Because of the 22cm isotopic shift between th . hydrogen and deuterium Lyman α (L α ) transitions, the ground state H atom produced by the dissociation will not absorb the deuterium L α wavelength. Therefore, when the reaction described by EQ. 2 is initiated by a "transfer laser" having a pulse length < 2nsec (the L fluoresence lifetime) , a substantial inversion in the D atom is formed o which provides an L^. laser at 1215A. We now determine the expected gain for such a L α laser. According - to an article entitled, "Nuclear Optical Processes in Atoms and Molecules Using Rare-Gas Halide Lasers", IEEE Journal of Quantum Electronics, Vol. QE-15, No. 5, May 1979, pp. 380-392, by W. K. Bischel, J. Bpkor, D. J. Kligler, and C. K. Rhodes, the two photon pumping of the H 2 (E,F 1 ∑ ) metastable state has a maximum obtainable excited state density N* given by: N* = ah v /(σ T (1+T /τ eff) ) (3) where N and N* are the ground and excited state number densities respectively, a is the two photon excitation coefficient, σ • is the photoionization cross section from the excited state, T p is the pump laser pulse width, and t e £ is the effective lifetime of the excited state. Since the largest N* is obtained when α , which is inversely proportional to the pump laser linewidth, is maximized it is advantageous to spectrally narrow the excimer laser. Using an excimer laser having a linewidth of 0.1cm provides α - 10~ cm 4 /W for the H 2 system. or an H 2 density of 10 molecules/cm^, one has τ eff = 10 ~9 sec, T p = 10~ 8 sec , and σ . - 2 x 10 18 cra 2 ; which provides that the H*(E,F λ ∑ ) density is ι i s - 2.4 x 10 XJ molecules/cm- 3 . The absorption length for the pump radiation is length = (N(H 2 )αi)-l = io 3 cm, laser intensity of I = 10 W/cm is chosen to optimize th H (E,F Σ_, ) pumping We calculate the cross section for laser induce collisional energy transfer to the HD (D λ ) target state, σ_, from the relation 2 2 2 σ c = lβπ J (μ 1 μ 2 u 3 E/( 3hΔv))Vh v p (4) where the μ^ are the relevant dipole matrix elements, E the applied electric field of the transfer laser, Δv is the effective detuning of the virtual level shown in FIG. 2, Pg is the dephasing or Weisskopf radius and v is the mean relative velocity of the molecules at collision. The dipole matrix elements, for the H 2 - HD collision syst are calculated using measured absorption cross sections a line widths for the H 2 molecule. For the H 2 (E,F -> B",D transitions μ- j _ - 2.2 x 10 esu-cm, an effective detuning Δv = 2 ~ - x 20cm -1 , for the H 2 (X -> B",D) transition μ 2 = 1.7 x 10" 19 esu-cm, and for the HD(X -> D) transition 3 ~ 2.5 x 10 esu-cm. The mean velocity fo collision at 300 K is v = 2.5 x 10 5 cm/sec. For p^ = 5 we compute σ = 1.2 x 10~ 2 ^(P/A) -1 cm 2 , where P/A is in units of W/cm. . The fraction of population stored in H*(E,F - 1 ∑ ) which is transferred to the target HD*(D π u ) level is calculated from collision arguments by assuming that the transfer occurs on a time scale that is short compared to the L α fluorescent lifetime and tha the HD*(D 1 π u ) state is predissociated with unity quant efficiency. The inversion density along the deuterium L α ' transition is equal to the fractional energy transfe and is given by: ΔN = N(HD)σ c P/Avτ (5) In this expression N(HD) is the number density of HD molecules and τ is the pulse length of the transfer o laser. We use a tunable dye laser at -6650A having a pulse energy of 10 j and a pulse length of τ = lnsec as the "transfer laser". A twice diffraction limited beam is focused to an area of 10 cm 2 over a confocal distance of 30cm to provide a "transfer laser" power density of P/A = 10 10 W/cm 2 . For an HD density of 10 18 molecules/cm 3 we obtain an inversion density AN = 7.2 x 10 atoms/cm 3 , which inversion density represents a 30 percent transfer of the stored H^ (E,F 1 ∑+ ) exci.tation. The single pass gain at the L α (1215A) transition is given by: Gain (1215A) = exp (ΔNσ ga±n L) (6) where gain is th e L , qam cross section and L = 30cm o is the confocal parameter. At 300 K, the Doppler width of lcm -1 for the L transition gives a gain cross section σ gain " 1 ' 2 x ID "13 2 Thus, for the parameters of the o above embodiment, one obtains- a single, pass gain at 1215A of e 26 -gπtEA OMPI A second embodiment of the present invention disclosed herein for energy transfer to an unbound molecular level is described by: Kr*(4p 5 6p) + HI(X 1 Σ + ) + hv(5-6μm) Kr(4p°) + HI* r(4p 6 ) + H(ls) + I*(6s p 1/2 ) (7) An energy level diagram corresponding to the reaction described by EQ. 7 is shown in FIG. 3. First, energy is stored in the Kr*(4p 6ρ) state of the krypton atom, the storage species in this embodiment, by two phot pumping of the 4p ground state of a krypton atom with th o 1930A radiation from an ArF excimer laser. Laser radiati from a "transfer laser" in the range of 5-6 ^ m , for examp a CO molecular laser, is applied to the Kr*. The resulti laser induced collisional energy transfer to the HI molecule causes the HI to be excited to an unbound molecular level. This HI dissociates to provide an inversion of the I(6s 2 P ι / ) resonance transition. (We note that laser emission may also be achieved from other resonance transitions but we will not discuss them furthe in order to avoid complexity which would detract from an understanding of th s., embodiment.) Since unbound dissociative levels typically have broad absorptions of t order of several hundred wavenumbers in width, the acceptable "transfer laser" wavelength may be chosen to optimally couple to desired virtual levels in such a manner as to reduce the virtual detuning, Δv . and thereby increase the overall collision cross section ~ c given in EQ. 4. We use EQ . 3 to calculate the maximum Kr*(4p 6p) population storage density. With an ArF excimer laser having a 0.1cm linewidth and a Kr number density of 10 atoms/cm 3 , we have α = 10~ 8 cm 4 /W, < ~ . - 10 _18 cm 2 , - - - p = 10 °sec, and e ς_ ~ 10 sec. This yields a Kr (4 6ρ) density of - 10 atoms/cm 3 . At a pump power density of 10 W/cra , the two photon absorption length is approximately 100 cm. We use EQ . 4 to compute the cross section for the laser induced- collisional energy transfer. Using values for the relevant dipole matrix elements- of μ i = 1 debye = 10~ 18 esu-cm, PQ =5A, ΔV - 2τr X 200cm "1 , and v = 3.5 x 10"cm/sec, we have a collision cross section of σ ~ 10 ~ 3(P/A) — x 1cιn ~ . This value is approximately two orders of magnitude larger than that computed for the H 2 - HD embodiment described hereinabove, due primarily to the larger dipole matrix elements. The l(6s 2 P ι / 2 ) inversion density due to this embodiment of the present invention is computed with EQ . 5. Available "transfer lasers" in the 5-5μm region can readily provide a lOmJ pulse in 5nsec, the I fluorescence lifetime, when focused to an area of 10 c over 5cm. At this power density of 2 x 10 8 W/cm 2 and an HI number density 10 17 molecules/cm , the fractional energy transfer to the HI* excited state is 3.5 percent. If the resulting I* excitation is distributed between the 6s 2 p and 6s -p levels according to their statistical weights, then approximately 1/9 of the energy will be transferred to the I(fis 2 P 2 ) level. The I ( s 2 P -> 5p 5 2 ^ /2^ inversi °n density is then 3.9 x 10 13 atoms/cm 3 . Allowing for 0.2cm -1 recoil broadening of the I resonance transition, the gain cross section is 3.2 x 10 13 cιr.2 # Using these pararne ters with EQ. 6, we compute a single pass gain of e at 1583 Cther embodiments similar to the second embodiment described hereinabove exist where energy from o 2480A KrF excimer laser pumps a Xe storage species by tw photon excitation and the second species is chosen from or HBr.

Claims: Claims 1. A laser apparatus comprising: a laser cavity (3) a storage species (50) and a second species (51) disposed in said chamber, said second species being a molecular material; excitation means (1) for exciting said storage species; characterized in that said apparatus further includes a laser source (2) of laser radiation and means (5) for applying said laser radiation to said storage species and said second species in said chamber, whereby said second species is excited to a higher state by a laser-induced collisional energy transfer interaction between said excited storage species and said second species and said highej state is a dissociative molecular state, whereupon said second species decays into a first and a second fragment, and at least one of said first or second fragments is excited to or decays to the upper level of a laser transition of said first or second fragment. 2. A laser apparatus in accordance with claim 1 CHARACTERIZED IN THAT said excitation means for exciting said storage species is a second laser source. 3. A laser apparatus in accordance with claim 1 CHARACTERIZED IN THAT said excitation means for exciting said storage species is an electric discharge. 4. A laser apparatus in accordance with claim 1 CHARACTERIZED IN THAT excitation means for exciting said storage species is an electron beam source. 5. A laser apparatus in accordance with claim 2 CHARACTERIZED IN THAT OMPI said storage species is E . and said second species is HD. 6. A laser apparatus in accordance with claim CHARACTERIZED IN THAT said second laser source is an ArF excimer laser for producing laser radiation substantially at 1930 which radiation excites said H 2 . 7. A laser apparatus in accordance with claim CHARACTERIZED IN THAT said laser source provides tunable radiatio o in the vicinity of 6650A. 8. A laser apparatus in accordance with claim CHARACTERIZED IN THAT said storage species is Kr and said second species is a HI. 9. A laser apparatus in accordance with claim CHARACTERIZED IN THAT said second laser source is an ArF excimer laser source for producing laser radiation substantially o 1930A and said laser source provides radiation in the ran of 5-6μm. 10. A laser apparatus in accordance with claim CHARACTERIZED IN THAT said laser source is a CO molecular laser. 11. A laser apparatus in accordance with claim CHARACTERIZED IN THAT said storage species is Xe and said second species is HCl. 12. A laser apparatus in accordance with claim CHARACTERIZED IN THAT said storage species is Xe and said second species is HBr. 13. A method of providing a laser transition CHARACTERIZED BY the steps of: _. exci ting (1 ) a . storage .speci es , whi ch s tora speci es is di sposed in a laser cavi ty ( 3) ; applying laser radiation (2) to said storage species and a second species, which second species is disposed in said laser cavity, whereby energy is transferred to said second species from said storage species by a laser induced collision, said energy transfer being sufficient to excite said second species to a dissociative molecular state, which dissociative state decays into a first and second fragment, which at least one of said first or second fragments is excited to or decays to the upper level of a laser transition of said first or second fragment.

Inventors: FREEMAN R, WHITE J
Applicants: WESTERN ELECTRIC CO, WESTERN ELECTRIC CO INC
Application year: 1981
Main classification: H01S3
IPC codes: A61K39, H01S3