Understanding Lasers: Chapter 7 Quiz
====================================

Source: Jeff Hecht, *Understanding Lasers: An Entry-Level Guide*, fourth
edition (2019), Chapter 7 quiz, printed pages 249--251.  The questions are
paraphrased.

Quick answers
-------------

.. csv-table::
   :header: "Question", "Answer"

   "1", "**b** in the printed key; **c** is also false"
   "2", "**b**, helium--cadmium"
   "3", "**d**, argon fluoride"
   "4", "**a**, approximately :math:`150`--:math:`1500\ \mathrm{nm}`"
   "5", "**c**"
   "6", "**b**, :math:`488.0` and :math:`514.5\ \mathrm{nm}`"
   "7", "**d**"
   "8", "**e**"
   "9", "**e**, all listed discharge geometries"
   "10", "**a**, carbon dioxide"
   "11", "**a**"
   "12", "**b**, HF"

Worked reasoning
----------------

#. **General gas-laser statement: b in the key, but the item is ambiguous.**
   Metal-vapour lasers use material that must be heated because it is not
   gaseous at room temperature, so **b** is false.  However, **c** is also
   false: mixtures such as helium--neon deliberately contain a second species
   that transfers excitation energy.  The item therefore has two defensible
   answers.

#. **Shortest continuous-wave output: b.**  A helium--cadmium laser can emit
   continuously in the ultraviolet.  The shorter-wavelength nitrogen and ArF
   choices normally operate as pulsed lasers.

#. **Shortest wavelength overall: d.**  ArF excimer emission near
   :math:`193\ \mathrm{nm}` is shorter than KrF, nitrogen, He--Cd, or argon-ion
   output in the choices.

#. **Electronic-transition span: a.**  Atomic and ionic gas-laser electronic
   transitions extend approximately from the deep ultraviolet near
   :math:`150\ \mathrm{nm}` to the near infrared near
   :math:`1500\ \mathrm{nm}`.  Molecular vibrational transitions extend much
   farther into the infrared but are not electronic transitions.

#. **Why retain a red He--Ne laser: c.**  It offers excellent spatial mode and
   beam quality.  Diode lasers are generally smaller, cheaper, and more
   efficient.

#. **Strong argon-ion lines: b.**  The familiar blue and green lines are
   :math:`488.0\ \mathrm{nm}` and :math:`514.5\ \mathrm{nm}`.

#. **After ignition of an argon discharge: d.**  High voltage initiates
   ionization; after breakdown, the voltage drops while a large current
   sustains the plasma and laser emission.

#. **Helium--cadmium distinction: e.**  Cadmium must be heated to supply metal
   vapour.  Neon is already gaseous under the tube's operating conditions.

#. **CO2 excitation geometries: e.**  CO2 lasers have been built with
   longitudinal, transverse, high-pressure, and flowing-gas discharges.

#. **Highest industrial gas-laser power: a.**  Carbon-dioxide lasers scale to
   high continuous power and have long been major cutting and welding tools.

#. **Excimer lower level: a.**  The excited dimer is bound, but its ground
   state is repulsive.  After photon emission it promptly dissociates, helping
   keep the lower laser level empty.

#. **Not an excimer molecule: b.**  ArF, KrF, XeCl, and XeF are excimer or
   exciplex laser species.  HF is the molecule in a chemical laser.