YELLOW SPRINGS INSTRUMENT CO.
1
0 2 ΡΡΜ Mhos ^, ^Λ Ohms «-^ Langley ^ Dissolved T ,, Oxygen etgs/cm 2 -sec D Vapor 'g Pressure s cal/cm 2 /sec If you want to measure any of these . . . temperature oxygen humidity conductivity light ... or control them, or record them, send for our catalog. We may have just the scientific instruments you need. YELLOW SPRINGS INSTRUMENT CO. YELLOW SPRINGS, OHIO 45387 Circle No. 179 on Readers' Service Card Report for Analytical Chemists 1 % . Since this light is coherent and nearly parallel, it may be focused to an area of dimensions approaching the wavelength of light. Although use is made of this property of the laser output in applications such as welding, the erratic behavior of this pulsed laser gives it limited use in problems of interest to chemists. One can partially overcome this dif- ficulty by Q-switching. The Q-Switched Laser If a shutter is placed inside the laser cavity and kept closed during the initial portion of the pumping pulse, the excited-state population in the laser rod will not be depleted by the lasing process which would normally occur, and hence will build up to a level greatly in excess of that attained without such a shut- ter. If, at the peak of this build-up of excited states, the shutter is sud- denly opened, the energy stored in the laser rod will be emitted very rapidly in a single, "giant" pulse (2). A cell containing a solution of a photo-bleachable dye is often used as the shutter, or Q-switch. To function in this capacity the dye must have an absorption band which overlaps the laser emission band. In this case, during the ini- tial stages of pumping all emission from the laser rod will pass through the Q-switch cell and will be atten- uated owing to the absorption of the dye. The dye concentration and the pumping power are adjusted so that the initial loss in the Q-switch off- sets the gain in the rod ; at this point the "shutter" is closed. However, as the excited-state population grows with continued pumping, the gain increases exponentially and eventually a condition of net ampli- fication is reached. At this point, the power of the light pulse starts to grow rapidly and becomes intense enough to bleach the Q-switch dye transition, opening the "shutter." The energy stored in the rod is then emitted over a period of typically 20—30 nsec duration. The total en- ergy output is reduced somewhat from that obtained in non-Q- switched operation, but since the pulse duration is shortened, the power is increased. The high power (tens to hundreds of megawatts) and short duration of the light out- put from Q-switched lasers have made it possible to study a variety of new phenomena. The high intensity has led, for example, to the observation of stim- ulated Raman scattering (3), har- monic generation (4), and two-pho- ton absorption (δ). The latter ef- fect has been detected by the ap- pearance of fluorescence—e.g., in anthracene (6)—or by chemical re- action of molecules transparent at the laser wavelength [polymeriza- tion of styrene (7), photodissocia- tion of chlorine (8) ]. The light flux available from Q-switched lasers can produce sufficient chemi- cal reaction in molecular beams to allow detection of photodecomposi- tion products and use has been made of laser pumping in an ele- gant series of molecular beam ex- periments (9). Upon irradiation by Q-switched lasers, certain crystals generate op- tical harmonics from the laser fun- damental frequency (4) ; with such "nonlinear" crystals one can obtain a 10% energy conversion of the laser frequency to its second har- monic. For ruby (14,400 cm - " 1 ) the second harmonic is at 28,800 cm' 1 ; for Nd 3 +: glass (9431 cm- 1 ) the second harmonic is at 18,863 cm" 1 , and the fourth harmonic (which can be obtained by frequency 7 doubling the second harmonic) is at 37,726 cm- 1 . These energies are sufficient- ly high to enable one to excite di- rectly by one-photon absorption most of the larger organic molecules. The absorption spectra of excited singlets and other short-lived inter- mediates of a number of molecules have been observed with the use of Q-switched pulses and second har- monic generation (10, 11). The Mode-Locked Laser While the use of Q-switched la- sers as an intense light source of nanosecond duration has led to much interesting research, this time scale is too long for detection and measurement of the most funda- mental molecular processes of inter- est to chemists. These are the processes occurring in a molecule very soon (within 10- 10 to ΙΟ -13 sec) after absorption of a photon. A technique, termed mode-locking, has made it possible to study these very rapid processes directly. The laser will support simultane- 22 A . ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
Transcript of YELLOW SPRINGS INSTRUMENT CO.
02ΡΡΜ
Mhos ^, ^Λ
Ohms «-̂ Langley ^
Dissolved T,, Oxygen
etgs/cm2-sec
D Vapor 'g Pressure s
cal/cm2/sec
If you want to measure any of these . . .
temperature oxygen humidity conductivity light
. . . or control them, or record them, send for our catalog. We may have just the scientific instruments you need.
YELLOW SPRINGS INSTRUMENT CO. YELLOW SPRINGS, OHIO 45387
Circle No. 179 on Readers' Service Card
Report for Analytical Chemists
1 %. Since this light is coherent and near ly parallel , it m a y be focused to an area of dimensions approaching the wavelength of light. Although use is made of this proper ty of the laser output in applications such as welding, the errat ic behavior of this pulsed laser gives it limited use in problems of interest to chemists. One can par t ia l ly overcome this difficulty by Q-switching.
The Q-Switched Laser
If a shut ter is placed inside the laser cavi ty and kept closed during the init ial portion of the pumping pulse, the excited-state populat ion in the laser rod will not be depleted by the lasing process which would normal ly occur, and hence will build up to a level greatly in excess of t ha t a t ta ined without such a shutter. If, at the peak of this bui ld-up of excited states, the shut ter is suddenly opened, the energy stored in the laser rod will be emitted very rapidly in a single, "g ian t " pulse (2). A cell containing a solution of a photo-bleachable dye is often used as the shutter , or Q-switch. To function in this capaci ty the dye must have an absorption band which overlaps the laser emission band. In this case, during the init ia l stages of pumping all emission from the laser rod will pass through the Q-switch cell and will be a t tenuated owing to the absorption of the dye. The dye concentration and the pumping power are adjusted so t ha t the initial loss in the Q-switch offsets the gain in the rod ; a t this point the "shut te r" is closed. However, as the excited-state populat ion grows with continued pumping, the gain increases exponentially and eventual ly a condition of net amplification is reached. At this point, the power of the light pulse s tar ts to grow rapidly and becomes intense enough to bleach the Q-switch dye transi t ion, opening the "shut ter ." T h e energy stored in the rod is then emitted over a period of typical ly 20—30 nsec durat ion. The to ta l energy output is reduced somewhat from tha t obtained in non-Q-switched operation, bu t since the pulse durat ion is shortened, the power is increased. T h e high power (tens to hundreds of megawat ts ) and short durat ion of the light outpu t from Q-switched lasers have
made it possible to s tudy a var ie ty of new phenomena.
The high intensity has led, for example, to the observation of s t imulated R a m a n scattering (3) , ha r monic generation (4) , and two-photon absorpt ion (δ). The la t ter effect has been detected by the ap pearance of fluorescence—e.g., in anthracene (6)—or by chemical reaction of molecules t ransparen t a t the laser wavelength [polymerization of s tyrene (7) , photodissociat ion of chlorine (8) ]. The light flux available from Q-switched lasers can produce sufficient chemical reaction in molecular beams to allow detection of photodecomposi-tion products and use has been made of laser pumping in an elegant series of molecular beam experiments (9).
Upon irradiat ion by Q-switched lasers, certain crystals generate optical harmonics from the laser fundamenta l frequency (4) ; with such "nonl inear" crystals one can obtain a 10% energy conversion of the laser frequency to its second harmonic. For ruby (14,400 cm-"1) the second harmonic is at 28,800 cm ' 1 ; for N d 3 + : glass (9431 cm-1) the second harmonic is a t 18,863 cm"1, and the fourth harmonic (which can be obtained by frequency7 doubling the second harmonic) is at 37,726 cm-1. These energies are sufficiently high to enable one to excite directly by one-photon absorption most of the larger organic molecules. T h e absorption spectra of excited singlets and other short-lived intermediates of a number of molecules have been observed with the use of Q-switched pulses and second har monic generation (10, 11).
The Mode-Locked Laser
While the use of Q-switched lasers as an intense light source of nanosecond durat ion has led to much interesting research, this t ime scale is too long for detection and measurement of the most fundamenta l molecular processes of interest to chemists. These are the processes occurring in a molecule very soon (within 10-10 to ΙΟ -13 sec) after absorption of a photon. A technique, termed mode-locking, has made it possible to s tudy these very rapid processes directly.
The laser will support s imul tane-
22 A . ANALYTICAL CHEMISTRY, VOL. 42 , NO. 14, DECEMBER 1970
