Experimental Raman (5)

Question 1

major advantages IR

Answer 1

Fundamental vibrations, molecular ID, libraries, instrumental simplicity

Question 2

major advantages Raman

Answer 2

narrower line widths, fibers and remote sampling, low frequency modes, H2O compatible, non-invasive sampling, enhancement mechanisms

Question 3

Disadvantages IR

Answer 3

broader line widths, sample difficulty, water interferences

Question 4

disadvantages Raman

Answer 4

low sensitivity, fluorescent interferences

Question 5

Main components that make up a generic Raman spectrometer

Answer 5

detection, collection, excitation

Question 6

Main characteristics of Raman lasers

Answer 6

wide range of excitation wavelengths possible (UV to Near IR)

Question 7

Why are CW lasers preferable to pulsed lasers?

Answer 7

pulsed laser peak power is too high, sample damage, photochemistry concerns

Question 8

Why is it necessary to filter the laser lines?

Answer 8

filter out extra atomic laser plasma lines for ion lasers

Question 9

Physical principles of ion lasers

Answer 9

DC electron discharge through low pressure Ar or Kr forms a plasma magnetically confined within resonant cavity; one end of cavity contains totally reflecting (1005) retro-reflector; other end of cavity contains partially reflecting (95%) output coupler; only a few transitions last with sufficient gain to superpower CW laser output

Question 10

What are common Ar wavelengths used as raman excitation lines?

Answer 10

514.5, 488, 351.1

Question 11

Common Kr wavelengths used as Raman excitation lines

Answer 11

676.4, 647.1, 413.1

Question 12

What are the main advantages of ion lasers?

Question 13

What are the main disadvantages of ion lasers?

Answer 13

most power used to create ions, not excite laser transition; only 0.01-0.05% incident power is converted to laser output; low efficiency means high electrical power and cooling requirements;

Question 14

power needed for small frame ion laser

Answer 14

208 V 3-phase, 10-15 kW total

Question 15

power needed large frame ion laser

Answer 15

408 V 3-phase, 50-60 kW total

Question 16

What are the physical principles of He-Ne lasers?

Answer 16

tasing transition is an Ne atom so power not consumed to create excited ions, energy transferred to Ne atoms before lasing, He electrons and ions carry current

Question 17

What is the He-Ne wavelength used as a raman excitation line?

Answer 17

632.8

Question 18

Advantages He-Ne lasers

Answer 18

lasing process much more efficient than ion lasers; ordinary 110 V power and air cooling sufficient; smaller and less expensive than ion lasers; laser wavelength well match to Si-based CCD Detectors

Question 19

Disadvantages He-Ne lasers

Answer 19

only single lasing wavelength supported; output optical power much lower for He-Ne lasers than for ion lasers (< 50 mW); must filter additional atomic emission lines

Question 20

What are the physical principles of near IR diode lasers?

Answer 20

doped semiconductors form a junction between p type (holes) and n type (electron) materials; electrons injected into conduction band of p type material, combine with holes, release energy equal to or exceeding band gap of material; recombination annihilates electron/hole pair, energy releases as light; pn junction functions as a light emitting diode, converts current to light

Question 21

What are the most common near IR diode wavelengths used as Raman excitation lines?

Answer 21

670 to 865

Question 22

What are the main advantages of near IR diode lasers?

Answer 22

high efficiency minimizes power and cooling requirements; very small footprint; wide wavelength range available in mid to near IR; inexpensive; useful power range of .1 to 1 watt

Question 23

disadvantages of diode lasers

Answer 23

broad gain curve results in wavelength drift; mode structure difficult to stabilize resulting in uncertainty in output wavelength and observed shift; high divergence = less easily filtered and focused