I will write another complementary answer because the top one unfortunately has some unit and physics inconsistencies.
First to answer the question:
have light gases like hydrogen or helium been explored for ion propulsion?
Depends on what you call "ion propulsion". If you mean gridded ion thrusters, then most probably no (see below). On the other hand, other types of electric propulsion concepts do use hydrogen and helium.
Electrothermal devices, such as resistojets and arcjets, generally have a maximum operating temperature due to material constraints. Their specific impulse can be very roughly estimated as $I_{sp} = \frac{1}{g_0} \sqrt{2 c_p T}$, where $g_0$ is the standard gravity acceleration, $c_p$ is the constant-pressure specific heat capacity and $T$ is the temperature. Since $c_p = C/m$, where $C$ is the heat capacity and $m$ is the propellant molecule mass, to increase the $I_{sp}$ to desirable levels (close or higher than 1000s), it is necessary both to increase $C$ by using atomic particles (decreasing the degrees of freedom of the particle), and decrease $m$.
Electromagnetic devices, such as the MPD were also largely operated with light propellants. MPDs specifically use it to increase the $I_{sp}$ at very high power levels. Lithium is particularly preferred to mitigate the erosion of the cathode which is one of the major problems of this technology.
Now to expand a bit on why it is not desirable to use Hydrogen or Helium (or any other light propellant) with gridded ion thrusters (or Hall thrusters).
Consider first that an ion thruster generates a collimated beam, with just one ion species. The power required to generate this beam is $P_b=I_bV_s$, where $I_b$ is the ion current and $V_s$ is the screen grid voltage.
The kinetic energy at which the ions leave the thruster is approximately $E_i=\frac{1}{2}m_iv_i^2$. Since the ions are accelerated by a electrostatic potential profile, the energy is equivalent to $E = q_iV_s$, since $V_s$ is approximately the potential at which is the ions are created.
The ion beam current can be rewritten as $I_b = \frac{q_i}{m_i}\dot{m}_i$, where $\dot{m}_i$ is the mass flow rate of ions. Using these four equations, the power can be rewritten as
$$ P_b = \left( \frac{q_i}{m_i} \dot{m}_i \right) \left( \frac{1}{2} \frac{m_i}{q_i} v_i^2 \right) = \frac{1}{2} \dot{m}_i v_i^2$$
which is exactly the formula for the "jet" or "beam" power given in textbooks. As you can see there is no guarantee that if you change the mass or charge of the ions the power consumed to accelerate the beam will change. The only thing that matters in this case is the mass flow rate and the speed that the ions exit the system.
On the other hand, we can rewrite the equation to evidence the thrust, as
$$P_b\approx \frac{1}{2} Tv_i \approx \frac{1}{2} T \sqrt{\frac{2q_iV_s}{m_i}}$$
In this case, for a fixed screen grid voltage $V_s$, to increase the thrust one has to either increase the ion mass or decrease its charge. So if you have a system that has a maximum voltage limitation (due to arcing for example) you have to increase the mass of ions to guarantee that you have a reasonable level of thrust thus decreasing your mission time.
Furthermore, as shown in the first chapter of Jahn's book, it is also important to notice that a higher $I_{sp}$ is not always better. There is an optimal specific impulse for a given mission which can be roughly estimated by
$$ \hat{I}_{sp} \approx \frac{1}{g_0} \sqrt{\frac{2 \eta \Delta t}{\alpha}} $$
where $g_0$ is the standard gravity acceleration, $\eta$ is the thruster efficiency, $\Delta t$ is the mission time and $\alpha$ is the ratio of the power processing subsystem and the power required by the thruster. This happens because as you increase the $I_{sp}$ you also increase the size of your power system, decreasing the overall payload mass of your spacecraft. This way, using a very light propellant like hydrogen to give you the maximum possible $I_{sp}$ may in fact just increase the overall required mass of your spacecraft.
The last point to consider is the amount of power you spend to create the ions. Consider that every time you ionize a neutral atom you spend an ionization energy $E_{iz}$. This way, the power you spend to ionize the ion mass flow rate is $P_{iz}= E_{iz}\frac{\dot{m}_i}{m_i}$. Using the expressions for both power terms (and excluding additional power contributions from the system), you have
$$ P = \dot{m}_i \left( \frac{v_i^2}{2} + \frac{E_{iz}}{m_i} \right) $$
The first ionization energy for xenon is 12.1 eV and of hydrogen is 13.6 eV. So you will spend around 150 times more power to ionize the same mass flow rate when using hydrogen as compared to xenon. Of course this is a very simplified analysis and more precise numbers would require a detailed analysis of the reactions and discharges using both propellants.
As commented by @asdfex, it is also important to notice that the ratio of the beam to ionization power will be constant in respect to the ion mass,
$$\frac{P_b}{P_{iz}} = \frac{qV_s}{E_{iz}}$$
This way, if you fix the screen grid voltage $V_s$ and let the average ion speed to vary, the only method to increase the amount of power going to the beam when compared to the ionization is by choosing propellants that have a smaller $E_{iz}$.